[HomeBrew] RE: Options For FM RX & TX Crystal Replacement?

[LIM,CHIN-LEONG (A-Malaysia,ex2)]

The plain vanilla synthesizer is stable enough for most application below 70
cm. Above 70 cm, you may need to temperature compensate the synthesizer's
reference crystal, eg. OCXO or TCXO. Additionally, the stability requirement
is different depending on the modulation - SSB being more demanding than FM.

DCkit (in US) and Minikits (in Australia) have synthesizer kits that may
work with your application. Both companies advertise their products on the
web. Their web addresses are listed below:-

http://www.dckits.com/

http://www.minikits.com.au/index.html

I have bought synthesizer kits from both company and I am generally
satisfied with their services. Generally, Minikit's synthesizer kits are
lower in price.

Whether your Tx chain can accept a square wave driving signal or not is
dependant on the circuit. Non-linear amplification (eg. Class C amps) are
running in the distorted mode anyway. The non-linear amp makes use of
extensive output filtering to clean up the signal just before the aerial.
However, a linear amplifier may be severely stressed if driven by a
non-sinusoidal signal, and may not have adequate output filtering to cope
with the harmonics.

For the Rx chain, it is equally acceptable to drive the downconverter/mixer
with either a sine wave or a square wave local oscillator signal. Spurious
responses to the LO's harmonics are usually suppressed by the filtering
stages prior to the mixer. In fact, some reference books say that there is
an advantage in driving a diode mixer stage with a square wave LO instead of
sine-wave, but that is the topic for another discussion.

Lim 9W2LC,
Penang, Malaysia.


>-----Original Message-----

>Date: Sun, 10 Feb 2002 21:24:30 -0600
>To: homebrew@mailman.qth.net
>From: Robin Midgett 

>Greetings,.
>I'm interested in options for substituting a frequency synthesized RF souce

>for a crystal in FM TX & RX strips. Why? The synthesized approach allows 
>easy frequency agility without purchasing new crystals. My questions are:
>Would a frequency synthesized source be stable enough to substitute for a 
>crystal oscillator?
>Is there an inexpensive synthesizer or PLL kit available for
experimentation ?
>Does the base oscillator in a FM RX or TX chain have to be a sinusoidal 
>waveform, or is a square wave or distorted sine waveform acceptable?
>What about the inexpensive clock oscillators modules? Some of them are 
>variable via a control voltage; stable enough? Square wave output
acceptable?

>Thanks & 73,
>Robin Midgett KB4IDC