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G8MNY > TECH 05.12.19 14:22l 552 Lines 28383 Bytes #999 (0) @ WW
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Mar 15)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Here are 22 modifications I have done to this popular scope adaptor design that
was first published in the RSGB's Radcom Technical Topics Apr 1988.
This design used just 3 ICs, 4 regulators & 1 transistor. A MC3356 is the 1st
osc mixer & log IF (most of the 94 transistors in the IC are not used), a MC602
(NE602) 2nd osc mixer, a TL084 quad op amp to do the sweep, & ñ12V 1A, +5 & +6V
100mA regs. (in 1989 a similar mark 2 design & PCB kit was published, with
bells & whistles, very complex with loads of 741s & calibrated switches).
LAYOUT
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄ¿Outer
³ÚÙMainsÀ¿ ÚÄÄÄÄÄÄÄ¿ Mains ³Box My one was very neatly UGLY
³³Transf-³ ³ PSU ³ ³ constructed (not by me) with
³À¿ormerÚÙ ³_______³ inner box ³ double sided PCB made box &
³ ÚÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ¿ ³ aluminium folded U top cover.
³ ³³ ÚÄ¿ r2 ÚÄÄ¿ ³ Op ÚÄ¿ ³³ ³
³ ³³ ³M³ ³()³ ³Amps³ ³ ³³ ³ r1=78L06 Then again with the RF bits
³ ³³ ³C³ ³()³ ³ ÀÄÙ ³³ ³ r2=78L05 in a 2nd bolted in PCB
³ ³³ ÀÄÙL2 ³()³ ÃÄÄÄÄÄÄÄÄÄÄ´³ ³ box with individual screen
³ ³³ L ÀÄÄÙ ³ ÚÄ¿ r1 ³³ ³ partitions & RF feed through
³ ³ÃÄÄÄÄÄÄ¿ IF1 ³ ÀÄÙ()2nd³³ ³ caps for lines in & out,
³ ³³L1 ³IFamp ³IF2 Mix ³³ ³ an RF tight fitting fingered
³ ÀÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÙ ³ aluminium lid.
³ ________ ____ Sweep ³
³ ³ Attn ³ ³Pot ³ ÚÄÄ¿ ÚÄ¿ ³
ÀÄÁÒuÄuÄuÁÄÒÄÁÄÂÂÄÁÄÒÄÁÂÂÁÄÄÁ=ÁÄÙ
RF X ÀÙ Y ÀÙ on/off
MY IMPROVED DESIGN
Input LPF Protect HF Trim 1st 1st IF 2nd 2nd IF IF Filter
Atten 0-90 Clip EQ Bias Mixer 145 Mixer 10.7 Amp 10.7
ÚÄÄÄ¿ ÚÄÄÄ¿ ÚÄÄÄ¿ ÚÄÄÄ¿ V ÚÄÄÄ¿ ÚÄÄÄ¿ ÚÄÄÄ¿ ÚÄÄÄ¿ ÚÄÄÄ¿ ÚÄÄÄ¿
o)Ä´ ³ ÃÄÄ´~~\ÃÄÄ´ ^vÃÄÄ´-Ä'ÃÄÄÁÄÄ´< >ÃÄÄ´_Û_ÃÄÄ´< >ÃÄÄ´_³_ÃÄÄ´³> ÃÄÄ´_³_ÃÄ¿
ÀÄÄÄÙ ÀÄÂÄÙ ÀÄÄÄÙ ÀÄÄÄÙ ÀÄÂÄÙ ÀÄÄÄÙ ÀÄÂÄÙ ÀÄÄÄÙ ÀÄÄÄÙ ÀÄÄÄÙ ³
ÚÄÄÄ¿ ³ ÚÄÁÄ¿ 1.5MHz ÚÄÁÄ¿ 50kHz 50kHz ³
/oÄÄÄ´ÁÁÁÃÄ>Ù 145-235³OSC³ ³OSC³ terminated for shape ³
ÀÄÄÄÙMarkers MHz ÀÄÂÄÙ ÀÄÄÄÙ ³
Vernier ÚÄÄÄ¿ ³Sweep 134.3MHz ³
Frequency<ÄÄÄÂÄÄ>ÄÂÄÄ´³> ÃÄÄÂÄÄÄ>ÄÄÄÙ \³/
Pot ³ ³ ÀÄÄÄÙ ³ ³
³ ³ ÚÄÄÄ¿ ³Corrected ³
ÚÄÄRamp ³ ÀÄÄ´_./ÃÄÄÙSweep ³
³ Sweep<ÄÙ ÀÄÄÄÙNFB Filter ³
³ Pot Log Amp & Detector 10.7 ³
X o)´ ÚÄÄÄ¿ ÚÄÄÄÄ¿ ÚÄÄÄ¿ Â ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄ¿ ³
S ÀÄÄ´ <³ÃÄÄ´ <³ ÃÄÄÄÄÄÄÄÄ´³> ÃÄ´< Sync ³Ú<ÃÂ<ÃÂ<ÃÂ<ÃÂ<³ÃÄÄÄ´_º_ÃÄÄÄÙ
C ÀÄÄÄÙ ³RampÃÄÄ<50Hz ÀÄÄÄÙ ³ clamp ³ 5 Detectors ³ ÀÄÄÄÙ
O Buffer ³Osc ÃÄ´ÃÄ¿ Flyback ³ ÀÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÙ 300kHz
P ÀÄÄÄÄÙ Á Amp ³ 10kHz ³sum Spurious
E ÚÄÄÄ¿1kHz ³ ÚÄÄÄ¿ ÚÄÄÄ¿ ³ Wall Filter
Y o)ÄÄÄ´~~\ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ<ÄÄÄÁÄÄ´_/~ÃÄÄÄ´~~\ÃÄÄÄ´
o\ ÀÄÂÄÙLP ÀÄÄÄÙ ÀÄÄÄÙ Y Cal
LPF ÀÄÄÄÙVideo Detector S Video Á
Switch Filter Correction Filter
After modifications here is the upgraded Specification:-
Frequency Range 200kHz-90MHz
Level flatness 200kHz-70MHz -3dB, 80 -6dB, 90-16dB
BNC 50ê input @ up to +20dBm (0.5W Max with all atten in),
70dB of input Attenuators 10, 20, & 40dB.
IF bandwidth 50kHz @ -20dB
Video Bandwidth 10kHz or 1kHz @ -3dB
Sensitivity +20dBuV (10uV) for 10dB/Noise
1st IC protected by clipper (not attenuator)
70dB of vertical Y log scale. (ñ2dB)
60dB mixer dynamic range before onset of mixer overload
BNC Y Scope output calibrated to 100mV/10dB, with negative flyback syncs.
Sweep rate 50Hz flyback locked to supply (ñ1Hz)
BNC X scope Ramp Output 20V P-P
5MHz RF Markers up to 90MHz
Vernier Dial Frequency readout, Accuracy ó 1MHz
No display of lower image sideband (below 0Hz)
210-254V 50-60Hz Mains Operation 10W.
CIRCUIT MODIFICATIONS:
INPUT
1/ There are 3 attenuators, the 10 & 20dB are accurately achievable with 1
double pole changeover switches separated by PCB screens. But the 40dB one is
not so easy, & needs some attention to detail, & an additional screening plate
between the connections to achieve it over the frequency range.
Note the 2k4 & 51ê are E24 series, but 2k2, & pairs of 100ê (* or 3x 150) work
quite well & may give higher dissipation if 500mW is put in. All Rs small types
& NOT WIRE WOUND! Very short leads are used on the 3 small switches.
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ ÚÄÄÄÄ2k4ÄÄÄÄ¿ ³ ³ ³
³ ³ oÄÄÄÄÄo ³ ³ oÄÄÄÄÄÄÄÄo ³ oÄÄÄÄÄÄÄo ³__ 50ê Coax
BNC o)ÄÁo/ | \oÁÄÄÄo/ \oÄÄo/ \oÄ)__ to filter
Input ³ o¿ | Úo ³ oÂÄ270êÄÂo ³ oÂÄ68êÄÂo ³
50ê ³ * 51ê | 51ê ³ 68ê 68ê ³ 100ê 100ê ³
ÀÄÄÄÄÄÁÄÁÄÁÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÙ
40dB 20dB 10dB
I found that the losses reduced by the odd dB at higher frequencies, this is
due to.. switch crosstalk capacitance, series R capacitance or low ê R
inductance, earth inductance etc.
2/ At the input to the low pass filter, add a 5MHz marker clock oscillator IC
mounted right beside the filter L. The output is loosely coupled by stray
capacitance with its short 1 cm lead for accurate frequency markers.
(The mk 2 SA has a marker already). The L1 is 5 turns wide spaced 5mm dia.
DC is via a push button.
Markers 14ÚÄÄÄÄÄ¿9 stray pick up protection
Äo\ÄÂÄ470êÄÂÄÄÄÂÄÄ´ 5MHzÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄ L1 clipper HF lift Pin 20
+12V ³ ³ ³ ³ OSC ³ from >ÂÄ(((()ÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄ´ÃÄÄ(((()ÄÄÂÄÄ> MC3356
=== _³_ === ³ IC ³ atten ÃÄÄ´ÃÄÄÄ´ _³_ _³_ ³ 10n 8turns ³/\ 250ê
u1³ 5V/_\' ³u1ÀÂÄÄÄÂÙ === 15p === \_/ /_\ 62ê 6mm === I/P Z
³ ³ ³ ³7 1³ 56p ³ 56p³ ³ ³ ³ /³ 20p
ÄÄÄÁÄÄÄÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄ
CLOCK IC LPF 2x1N4148 Term
3/ Change the low pass filter termination to 62ê as it is in parallel with 250ê
IC input Z. My filter is 3dB down @ 90MHz with about 30dB rejection above that.
4/ Add RF clipping diodes across termination R to protect IC1. These do not
conduct at all, for on screen signal levels. But do stop you blowing up the IC
with silly signals (e.g. from a handheld), but the input attenuator is
unprotected!
5/ The conditioned signals from the attenuator & VHF wall filter is fed through
an L & C trimmer for best HF level @ 70MHz into the 250ê input Z of the mixer
IC1 (1st « of MC3356 IC). The IC is not designed for the Local Osc @ so high
frequency, so there is some drop off of sensitivity @ 80MHz without this tweak.
1st MIXER in MC3356.
A 2:1 step up ferrite transformer & no termination resistor will give 6dB more
gain, but no transformer is flat over 0.5 - 90MHz range so this is not used.
The mixer has a gain of about 5dB.
The osc provides 2 out of phase outputs (one with no RF!) that are buffered to
drive the Gilbert mixer cell (unbalanced), which has one RF input & 1 output.
Tuned cct OSC BUFFERS MIXER CELL
+12VÂÄÄÄÄÄÂÄÄÄÄÄ 4oÄÄÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ¿ ÚÄ +12V
=== ) L2 ³ ³/ ³ ³ ³ ³ 4.3uH )
10n_³_ ÃÄÄÄÄÄ 3oÄÂÄÄÄ)Ä10kÄ´ ³ ³ ³ IF ³ L3 ) 1n
ÚÄÄÄ´ \³ ³ ³\e ³/ ³ ÚÄÄÄÄÄÄÄ)ÄÄÄÂÄÄÄÄÄÄ)Äo5 ÄÄÄÁÄ´ÃÄ>
=== ³ ÃÄÁÄ10kÄÄÄ)Ä´ ³ ³ ³ ³ ³ to 145MHz
/_\ === 2p2 e/³ ³ ³\e ³/ \³ ³/ \³ 50k IF Filter
Sweep ³ ÃÄÄÄÄÄ 2oÄ´ ³ ÃÄ´ ÃÄÂÄ´ ÃÄ¿ ³
>Ä10kÄ´ ³ ³ ³ ³ ³\e e/³ ³ ³\e e/³ ³ ³
³ ³ ³ ³ ³ ÀÄÄÂÙ ³ ÀÂÄÄÙ ³ ³
³ === 5p ³ ÃÄÄÄ)ÄÄÄÄÄÄ)ÄÄÄÄÙ ³ ³ ³
1n === ³ ³ ³ ÃÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÙ ³
³ ³ RF in ³ ³ ³ ³/ \³ ³
From>Ä)ÄÄÄ)ÄÄÂÄ 20oÄ)ÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÄ)ÄÄÂÄ´ ÃÄÂÄ10kÄ´
LPF ³ ³ ³ ³ ³ ³ ³ ³\e e/³ ³ ³
³ ³ ³ ³ ³ ³ ³ ÃÄÄÄÄÄÄÄÄÄ´ ³ ³
³ ³ ³ ³ ³ ³ ÀÄÄÄ)ÄÄÄÄÄÄÄÄÄ)ÄÄÄ)Ä10kÄ´
³ ³ ³ ³ ³ ³ ³ ³ ³ _³_
³ ³ 1M 50k 50k 50k 330ê 330ê === \_/
³ ³ Preset ³ ³ ³ ³ ³ ³20p _³_
³ ³ ³ ³ ³ ³ ³ ³ ³ \_/
0V ÄÄÁÄÄÄÁÄÄÁÄ 1oÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÙ
L2 is a spread out coil 5 turns 5mm dia. The 12V is well decoupled with 10n @
L3. The original Varicap was a MV209.
N.B. never accidentally put a earth on pin 2, as this will destroy the OSC NPN!
6/ To get the last drop of balance out the 1st mixer, I found a 1M preset from
RF input to ground could give a slight improvement in balance & reduce a 2nd
harmonic of a pure RF signal by 2dB.
dB 0Hz
+70´³
+60´³ .Fo If the balance is the other
+50´³ ^ ³ way, try the pot to +12V.
+40´³ | ³ Adjust 1M
+30´º ~60dB ³ for Min Use a well filtered Osc (2-30MHz)
+20´º | º \|/ so the 2nd harmonic > -60dBc
+10´º v º 2Fo Noise
0´ÐÄÄÄÄÄÄÄÄßÄÄÄÄÄÄÄÁÄÄÄÄ Floor
7/ The varicap tuned first local VHF oscillator's range has extended to tune
from 145MHz to 235MHz by adding further UHF Varicaps across the initial one &
stretching/reducing the osc L2. This may depend on the varicap used & stray C.
8/ The local oscillator coil is adjusted to give 0Hz line (e.g. osc = 145MHz)
when the FREQUENCY control vernier is set to 0 (mechanically near the -12V end
of the pot, so that tuning sweep voltage after the amp is near to +12V). This
also stops the unwanted image side of the 0Hz from being displayed & causing
confusion. The 22uF bipolar cap removes any scratchyness in the freq pot.
+12VÄÄÄÄÄÄÄÄÄÄ¿
FREQ<ÄÄÄÄÄÄÄÄÂÄÄ>Tuning ³ 0Hz
10k POT ³ DC ³ ³
³ ===22u ³ No ³ 5MHz
80MHz CAL ³Bipolar ³Inverted³ ³ Markers
5k PRESET _³_ ³Display º | ³ ³
-12VÄÄÄ´<ÃÄÄÄÄÙ ³ \³/ º ³ ³ ³ ³ ³
Thermal ³________º_³_³_³_³_³_
compensation. (See 21/ & 22/)
1st IF FILTER
9/ Mine used a standard 50ê 2M three pole TOKO 144-146MHz filter, (the original
article said to build your own). The TOKO one can be modified from a bandwidth
of about 3MHz (-10dB) with 3 peaks, to a single peak 1.5MHz wide, by adding 2
small metal shielding strips (6mm x 8mm) to cover the 2 apertures between the 3
coil sections, BUT this is fiddly to do!
Underside view After mod
ÚÄÄÄÂÄÄÄÂÄÄÄ¿ ÚÄÄÄÂÄÄÄÂÄÄÄ¿
³( ) ( ) ( )³ ³( )³( )³( )³
ÀÄÄÄÁÄÄÄÁÄÄÄÙ ÀÄÄÄÁÄÄÄÁÄÄÄÙ
To see the 1st IF response on its own on the display, to tune it up, temporally
remove connections to the 2nd IF filter & bypass it with a bridging 1nF cap &
feed a carrier in (marker).
_ _ _ ..
³ / \./ \./ \ ³ / \
³ | | ³ | |
³ ³ ³ ³ ³ ³
³ | | ³ | |
³_./ 3MHz \._ ³____./ 1.5MHz \.__
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
Once the 1st IF's bandwidth is reduced then the number of signals reaching the
2nd mixer is greatly reduced & hence this reduces the amount of unwanted
distortions & close in mixing products being displayed.
2nd MIXER.
This uses a NE602 osc & a balanced Gilbert cell mixer (used unbalanced) with
similar internal circuit to that of the MC3356 RF part, it runs on its own +6V
regulator & RF decoupled. Mixer gain is about 17dB.
+6VÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄ¿ * see /11 for value
=== 8t ³,/\ ³ *³ === 10n
1n_³_ 5mm (| ³8 430ê _³_
/(| 6ÚÄÄÄÄÁÄÄÄÄ¿ ³
134.3MHzÃÄÄÄÄÄ´ ³4 ³ to 10.7MHz
3p9 === 7³ NE ÃÄÄÄÁÄÄÄÄÄÄ>ceramic
145MHz ÃÄÄÄÄÄ´ 602 ³ filter
IF >ÄÄÄ´ÃÄÄÄ)ÄÄÄÄÄ´ ³ N.B. as with 1st
1n ³ 1ÀÂÄÄÄÂÄÄÄÂÙ osc, an accidental
=== ³2 ³3 ³5 earth on pin 7 will
6p8 ³ 1n=== ³ ===1n destroy the osc NPN.
ÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÄ
10/ The VHF oscillator in the 602 should be run on a lower frequency (core "in"
position) to the 1st IF, this is to reduce spurious images. e.g. 10.7MHz 2nd
IF needs 134.3MHz, (or a 6MHz IF needs 139MHz). Due to the ferrite core this
osc is susceptible to changes in magnetic fields, so mains transformer flux can
be a problem for "zoomed in" stability!
2nd IF FILTER
11/ The 2nd mixer's output has in internal pull up of 1k5 to so get 330ê source
impedance for the filter a 430ê on pin 4 determines the 2nd IF filter source
impedance. (This is not applicable to the Mark 2 with narrow filter option.)
³ .-Ä-Ä-. ³ .--. If the filters are deliberately
³ Þ Ý ³ / \ mismatched then they can give
³ Ý COMMS Þ ³ | | a better analyser friendly
³ | Filter | ³ | | "rounded peak response" rather
³_./ 50kHz \._ ³_./ \._ than the flat topped ringy
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ edges of communication filters.
Correctly terminated Mis Terminated
RINGY FILTER sweep friendly filter
To find the best values for your filters use small 1k presets to source &
terminate the filters to see this effect on the display of a carrier, find the
optimum value for best IF shape & then replace the presets with nearest fixed
values.
+12VÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄ¿
=== u1 ³ IF ³
+6V _³_ ³ AMP 470ê
2nd Pin 8<Ä´ 220k ÚÄÄÄÄ´
Mixer 430ê ³ ³/ ³
602 Pin 4>ÄÁÄ¿ ÚÄÄÄÄÄÄÄÁÄÄ´ T1 ³ ÚÄÄÄÄ¿ ÚÄÄÄÂÄÄÄÄÄÄ>Pin 7 To
Pin _³_ _³_ BFX90³\e _³_ _³_ _³_ _³_ 330ê MC3356
3 ÚÄÄÄÄÄ¿ ³ ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ ÃÄÄÄÄÄÄ>Pin 9 Log
³ 50kHzÀÄÄÄÄÄÙ 100ê ÀÄÄÄÄÄÙ ÀÄÄÄÄÄÙ ÀÄ´ÃÄÂÄ>Pin 8 Detector
³ 10.7 ÄÄÄÂÄÄÄ Preset ÄÄÄÂÄÄÄ ÄÄÄÂÄÄÄ ===
ÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄ>Pin 19
1st filter 2nd Wide filter
Adjust the 100ê gain preset for the optimum noise floor that can just be seen.
12/ Adding an inter filter buffer stage using a single transistor T1 adds some
preset gain for setting the overall noise floor of the analyser, as well as
matching into a 2nd filter. This filter is terminated by the input load on the
detector 330ê. Two 50kHz 10.7MHz ceramic filters provide a reasonable
compromise of selectivity for sweeping 0-80MHz @ 50Hz without too much ringing
distorting & loss of the peaks levels (up to 10dB) while still looking good in
close "zoomed in" sweeps. However a 3rd filter was put in tandem with the 2nd
filter to clean up poor filter skirt rejection of my particular narrow filters
@ 7MHz!
LOG DETECTOR
13/ The detector (S meter output) uses 5 IF amps & detectors (with limiters) to
obtain log response & it is quite accurate for over 40dB range. But ignoring
the slight overload in the mixers this range can be extended on the display, by
increasing the gain calibration preset (1k preset now 2k2), & then adding a non
linear correction attenuator with diodes D1 (Schotky/Ge) & D2 (Si) to give 30%
stretch @ the highest & lowest levels where the detector has lower sensitivity.
Ge 1.0 ´Output from _.-'
MC3356 pin ÚÄ´>ÃÄ¿ Y to .9 ´Detector .-'
LOG 14>ÄÂÄÄÄ´ ÃÄÄÄÄÂÄÄÄÄ> scope .8 ´ .'
DETECTOR ³ ÃÄ10kÄ´ ³ .7 ´ .'
2n2³ 680ê ³ ³ T2 .6 ´ .' S
Video === ³ 22k \³ Timebase .5 ´ .' Correction
Filter ³ 2k2 _³_ ÃÄ<Sync/ .4 ´ _.-'
³ Y CAL \_/Si e/³ Blanking .3 ´_.-'
Pin 11>ÄÄ´ÃÄÁÄÄÄÁÄÄÂÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÄ .2 ´
Lim out 1n _³_ .1 ´ Display
is RF 0v ÅÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂ>output
grounded 0 10 20 30 40 50 60 70 dB
With this correction you can get good display linearity to 70dB. Easily tested
with input attenuator & a signal generator to see equal height 10dB steps.
14/ The sensitivity is set by the Y Calibration preset to give 100mV/10dB. A
2.2nF capacitor limits the Y video bandwidth to about 10kHz (50%), but having
hardly any degradation of pulse height at the widest sweep range.
VIDEO FILTER
15/ For some applications lower video bandwidth is needed to reduce noise, I
added a 33nF switched across the Y output incorporated with the above mod, to
give about 1kHz Y bandwidth (50%).
Ä10kÄÄÄÂÄ>Y to
³ scope
o\Ä´ÃÙ
_³_ 33nF
It needs to be switchable as it causes the output to lie about the fine detail
with wide sweeps.
POWER SUPPLY
16/ The hot +12V regulator has been heatsinked, & the + rail input smoothing
capacitor increased from 680uF to 2m2. Transformer & rectifier pulse currents
wiring & layout have been kept away from the regulators as far as possible to
reduce supply hum ripple pickup.
Heatsink
4x 1N4001 +17V ÚÄÄÄÄ¿
ÚÄÄÄÂÄ´>ÃÄÂÄÄÄÄÄÄÄÂÄÄ´7812ÃÄÂÄÄÄÄÄÄ> +12V Much larger output
L >Äo/ oÄÂÄ¿ ³ _³_ _³_ 2m2 ³+ ÀÄÂÄÄÙ+³330u 200mA caps have been
100k )º( /_\ /_\ 25v=== ³ ===16v used to reduce
240V ³ )º(___ ³ ___ ³ ______³____³____³_____\ 0V the last remnants
NEON )º( ³ ³ 680u³+ ³ ³+ / of hum & noise.
³ )º( ³ ³ 25v=== ³ ===330u This is most
N >ÄÄÄÄÄÄÁÄÙ ³ ³ ÀÄÄÄÄÄ)ÄÄÄÄÄÄ´ ÚÄÁÄÄ¿ ³ 16v important
³ ÀÄÄÄÄÄÄÄÄÂÁÄÄ´<ÃÄÁÄÄÄÄ´7912ÃÄÁÄÄÄÄÄ> -12V for close in
E >ÄÄÂÄÄÄÄÄÄÄÙ 14-0-14 ³ -19V ÀÄÄÄÄÙ 20mA stability.
ÀÄ> 0V 0.3A ÀÄ>50Hz
The mains transformer has also been varnished to reduce acoustic hum & an outer
copper short circuit added to reduce magnetic fields that can affect the 2nd
osc stability. The other 2 low power regulators +6V for 2nd osc, & +5V for Log
amp, are placed near those circuits for best noise/voltage error rejection.
17/ A 50Hz synchronisation line is provided for ramp timebase locking. This is
important for close "zoomed in" stability of the sweep.
RAMP GENERATOR
In the simple mark 1 design, it uses 4 operational amplifiers IC3 (e.g. TL084)
that run on the ñ12V. The origin ircuit produced a symmetrical 500Hz ramp up
& down oscillator which was far too fast for wide sweeps & half the time was
wasted during the flyback. Mod 18/ solves this.
18/ IC3a forms a 50Hz ramp oscillator with the 100k & 12k in parallel during
flyback due to the diode D3, & a 1uF timing capacitor to give close to mains
frequency. Then a small injection of 50Hz from the mains transformer alters the
flyback time (D3, 12k & 1uF) to cause lock up to mains frequency. This method
ensures constant sweep MHz rate & the mains lock ensure a stable display even
with some sweep hum present when zoomed close in.
Ä¿ÚÄÄÄ¿
50Hz 12VAC >ÄÄ100kÄÂÄ12kÄÂÄÄÂÄÄÄÄÄÄÄ¿ ÀÙ ÀÙ _
from bridge _³_ ³ 100k ³ Ú¿ _³ \____
MAINS D3 /_\ 100k ³ ³ ÄÙÀÄÄ
LOCKED /ÃÄÄÄÄÄÄÄÄ´ ³ ÃÄÄ´\ ³ - ÚÄÄ´>ÃÄÂÄ22kÄ>Y Blanking
50Hz <ÄÄÄÄÄÂÄÄÄÄ<'ñ³IC3b ³ ³ ³ ³ñ`>ÄÁÄÄ´\ ³ ³ Transistor
RAMP,. ³ n47 `\ÃÄÂÄÄ¿ ÃÄÄÄÄÄÁÄÄ)ÄÄ´/' +³ `>ÄÄÁÄÄ2k2Ä´ T2
,/ ³ ÃÄ´ÃÄÄÄÄÄÄ´ ³ ³ ³ IC3a ÚÄÄ´/' ³
,/ ³,/ ÀÄÄÄ10kÄÄÄÙ 1k2 ===1u 10k ³ IC3d u1===
' ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄ
Buffer x9 ,/'³,/'³ 50Hz Ramp Osc Sweep Settle Delay
19/ IC3d buffers &
inverts the banking dB 0Hz /|\
pulse & it is +70´ ³ | | + 0.9V
lengthened with CR +60´ ³ Sweep |
& a diode before it +50´ ³ Centre 100mV
drives blanking +40´ ³ | /10dB
transistor T2. This +30´ º |
eliminates any sweep +20´ º |
folding due to VHF osc +10´ º Noise Floor \|/
sweep settling delay 0´ ÚßÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ + 0.2V
with RF sweep filtering SYNCS³ SWEEP ^ ³
to be masked. And the ÄÄÄÙ | ÀÄÄÄÙ - 0V
banking also gives Scope^ Scope
the scope a 0V sync Trigger Flyback
pulse to lock to. < - - - - 20mS - - - - ->
20/ IC3c is the sweep correction amplifier, this amplifies the selected sweep
width together with the centre frequency DC, then corrects for VHF oscillator
varicap frequency control non linearity by pre-distorting the ramp waveform
with 5 gain changes using 4 diodes, D4-D6 & ZD2 zener.
-12V>Ä3k3ÄÄÄÂÄÄ2k2ÄÄ<+12V MHz Tune
10 Turn 390k 240´Ramp _
Tune<Ä68k¿ R1 230´Input _..--''~
Pot ÃÄÄ27kÄÄÂÄ´<ÃÄ´ 220´ Five _.--'~ R4
³ 27k _³_ 210´ Slopes _.-' R3
Ramp ³ ÚÄ´\ ³ /_\ To VHF 200´ .-'
³ ³ ³ ³ñ`>ÁÄÄÄÄÄ)ÄÄÄÄÄ>Osc 190´ .-' R2
Sweep ÃÄ)Ä´/'IC3c ³ Varicap 180´ .'
Pot<Ä68kÙ ³ ³ 2.7V 170´ .' R1
10k ³ ÃÄ´>ÃÄÄÂÄ´<ÃÄ¿ 160´ ;
³ ³ R2 47k Ù 15k 4k7 150´: Varicap Volts (ref to +12V)
ÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÁ 140ÅÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂ
R3 R4 +12 9 6 3 0 -3 -6 -9 -12
The R1-4 values used for the gain corrections are set up using the marker to
give even spacings on the display.
0Hz
³
³ . Even spaced 5MHz markers
³ ³ . ³ | | . . .
º ³ ³ ³ ³ ³ | ³ | ³ | ³ | ³ | ³ | ³
º ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ ³ 90
_º_³_³_³_³_³_³_³_³_³_³_³_³_³_³_³_³_³_³_
5 15 25 35 45 55 65 75 85 MHz
FREQUENCY CONTROL
21/
+12VÄÄÄÄÄÄÄÄÄÄÄ¿
FREQ<ÄÄÄÄÄÄÄÄÂÄÄ>Tuning The multiturn vernier
10k POT ³ DC centre frequency control
³ === 22uF has a 22uF bipolar
80MHz CAL ³Bipolar! capacitor to ground
5k PRESET _³_ (or elect to +12V near the ICs)
-12VÄÄ´<ÃÄÄÄÄÄÄÙ //// to remove any pot scratchiness.
Thermal
compensation.
22/ A multiturn preset pot on the positive rail of the control is added to
calibrate 80MHz position on the vernier scale. A diode in series gives some
temperature drift compensation. Together with the correction circuit of IC3c
fairly accurate frequency readouts are possible on the vernier scale. 0-90MHz.
I N U S E
IMAGES
Other than the 0 Hz line, there is only one unwanted image @ 10.7MHz, it is at
a low level & its appearance depends on the IF gain setting. It is due to the
second IF detector being in the same IC as the RF input section!
dB 0Hz
+70´ ³
+60´ ³ VHF images are all well
+50´ ³ down due to the VHF LPF
+40´ º & the chip sensitivity
+30´ º cutting off as well as
+20´ º the input filter & double
+10´ º Noise screened box.
0ÅÄßÄÄÄÄÄÄ^ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Floor
10.7MHz
OVERLOADS
These can be seen as higher levels of harmonics increasing at a greater rate
than the fundamental. e.g. a 10dB increase in level, causes the fundamental to
increase by 10dB (1 division), but the 2nd harmonic increases by 15 to 30dB!
dB 0Hz dB 0Hz
+70´³ +70´³ Fo
+60´³ Fo +60´³ ^ ³
+50´³ /|\ ³ +50´³ | ³ Mixer
+40´³ | ³ +40´³ | ³ Generated
+30´º <60dB³ +30´º >60dB³ Harmonics
+20´º | º +20³º | º 2Fo
+10´º v º 2Fo 3Fo Noise +10´º v º ³ 3Fo
0ÁßÄÄÄÄÄÄÐÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Floor 0´ßÄÄÄÄÄÄßÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄ
Filtered Osc test Filtered Osc test
OVER LOADING 1st MIXER
They can also be detected as unwanted sidebands around the markers too.
dB dB
+70´ Signal +70´
+60´ ³ +60´
+50´ ³ +50´
+40´ ³ Marker +40´
+30´ ³ ³ +30´ Base Noise Floor
+20´ SÄM º ³ S+M +20³ Raised __
+10´ Mix º ³ Mix +10´ __..--""~~
0´ÄÄÁÄÄÄÄßÄÄÄÄÐÄÄÄÄÁÄÄÄÄ 0´ÄÄ""~~
POSSIBLY OVER LOADING MIXER GROSS OVERLOAD
Other signs of overload is a raised noise floor.
CLOSE IN OVERLOADS
These are much the same as above, but occur when the 2nd mixer sees 2 large
signals passing through the 1st IF filter. So if the narrowing of that filter
has been done, strong signals will need to be closer than 1 MHz to suffer this
problem. (e.g. using the analyser closer than 1 MHz from 0 Hz reduces dynamic
range due to the increased noise floor from its' own 2 oscillators)
FILTER NOISE SIDEBANDS
dB With large carriers are looked
+70´ at close in, you will see noise
+60´ /~\ sidebands (phase noise) added
+50´ ³ ³ to the filter response, this is
+40´ Ý Þ normal for this sort of analyser.
+30´ Þ Ý
+20´ Sideband ÜÝ ÞÜ Sideband Lower frequency Y display filtering
+10´ Noise _ÜÛß ßÛÜ_ Noise can mask this, but at the cost of
0´ÄÄÄÄÄͼßßß <50kHz> ßßßÈÍÄÄÄÄÄÄÄ peak pulse height accuracy.
SWEEP NOISE
Some of this phase noise can be noisy sweep amps, as the S/N needed on the VHF
osc will be >120dB, e.g. 24V max sweep & < 24uV of noise! I have used active
sweep filtering on some analysers to overcome this failing where the sweep rate
is low & a CR filter after the last opamp does reduce the HF noise sent to the
oscillator.
ÚÄ´>ÃÄ¿ The diode direction (depends on circuit)
³\ ³ ³ ensures the flyback charges up the cap
OP ³ `>ÄÁÄÄRÄÄÁÂÄÄÄÄ>VHF OSC voltage quickly, so there is little cramping
amp ³/' === C at the start of the sweep.
ÄÄÁÄÄ e.g. Xc = R (-3dB) @ 10x sweep freq.
See my tech buls "Spectrum Harmonic Demo circuit", "RF Directional Coupler", &
"Analyser Takeda Riken TR4122B".
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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