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ZL2WRW > PACKET 04.04.13 21:03l 18 Lines 1035 Bytes #999 (0) @ WW
BID : 000475073PZT
Read: GUEST
Subj: FM modulator linearisation?
Path: IZ3LSV<IW0QNL<F6IQF<IK6ZDE<VE3UIL<GB7MAX<GB7PZT
Sent: 130315/1928Z @:GB7PZT.#24.GBR.EU [Kidderminster] #:47500 XSERV410h
Date: Fri, 15 Mar 2013 19:28:53 +0000
From: zl2wrw@gb7pzt.#24.gbr.eu (Ross)
To: packet@ww
Subject: FM modulator linearisation?
Hi,
I am converting a Tait T555 (UHF) for 9k6 packet on 70cm.
This radio uses a PLL frequency synthesiser, so I am replacing the 12.8 MHz PLL reference oscillator crystal with a 12.8MHz VCTCXO so that I can implement dual point FM modulation (the other modulation point is the OEM modulation injection point into the TX VCO).
I have plotted the frequency response vs control voltage of the VCTCXO and unfortunately it is non-linear. This will result in a peak negative deviation that is about 14% greater than the peak positive deviation (for low frequencies within the PLL loop filter pass band). This will add a strong even-order harmonic component to modulation frequencies that fall within the PLL loop filter pass band.
Has anyone attempted to normalise the non-linearity of a TCVCXO?, or does the non-linearity and resultant distortion not really matter?
73 ZL2WRW
Ross Whenmouth
ZL2WRW @ GB7PZT
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