EMRFD Message Archive 2694

Message Date From Subject
2694 2009-02-13 06:20:38 Vojtěch Bubník oscillator stability, VFO-BFO stability?
Hi gang.

I have a question on frequency stability of crystal oscillators. I am
experimenting with ATS-3b transceiver and digital modes. The transceiver has a
simple ne612 superhet receiver. The mixer is clocked with DDS, BFO is clocked
with 4.9MHz quartz connected to NE612. DDS is clocked with 50MHz canned
oscillator. No clock mutiplication is done inside the DDS. The transceiver is as
simple as it could be. DDS output is squared by precision comparator inside the
DDS chip after low pass filtering and the same square wave drives a buffer and
BF170 mosfet PA into saturation.

I managed to extend the original firmware to modulate various digital modes by
programming the DDS, namely BPSK31/BPSK64 and various multi tone modulations
like RTTY, MFSK16 or Olivia. To make the multi tone modes with low modulation
speed and narrow spacing of tones work, one needs to know the offset between VFO
and BFO quite exactly so if he selects a frequency on the waterfall, he will
really answer on the same frequency. MFSK16 tone spacing is 15Hz, so one wants
to tune to about +-3Hz from the center of the other party. This is not an issue
with classic SSB transceiver, which is using the same VFO and BFO for RX and TX
as well.

If the oscillators are shifting slowly, the operator sees it on the waterfall
and the software AFC is able to cope with it. In case of SSB transceiver both
parties will be frequency synchronized. But in the case of the ATS-3b, the RX/TX
offset will be dependent on delta of abs(VFO-BFO). The VFO frequency instability
is a fraction of the canned 50MHz oscillator instability. For example,
generating 1MHz VFO will produce 1/50 instability of the canned oscillator. The
BFO frequency instability depends on the crystal quality, quality of the passive
network around the oscillator and the NE612 chip. Also the BFO crystal is the
same as those of the IF ladder filter and it is pulled below the filter with
33uH coil wound on a ferit toroid core.

I know roughly the effects of various components on frequency precision, but I
am lacking a feeling for proportions. I could measure audio beats using sound
card with sub Hz precision. I don't have any precision frequency source though,
so the audio method only shows there are some frequency shifts, but not the
source. I would greatly appreciate someone experienced to give me an advice what
VFO-BFO shift may I expect if the temperature swings between 0 and 30
centigrades. What effect has the passive network of BFO, namely the pulling
inductor? What inductor shall I use to minimize the temperature effects? I wound
20 turns on FT37-61 core.

73, Vojtech OK1IAK
2700 2009-02-14 09:10:18 joop_l Re: oscillator stability, VFO-BFO stability?
2702 2009-02-14 16:35:06 John Kolb Re: oscillator stability, VFO-BFO stability?
I remember some of the coils we used at Southcom had as many as 110 turns
on a T68 toroid. 28 ga wire as I recall, but perhaps smaller.


At 09:10 AM 2/14/2009, you wrote:

>33uH, that is a lot. How much did you pull it down? more than 5KHz or so?
>The ferrite #61 material seems to have a tempco of 1000ppm/degree
>celcius. You really should try not to use ferrite for tuning. Iron
>powder is better, but your 33uH might make it impractical. Often T68-6
>is used for stable VFO's (tempco 35ppm/c) but it would require 84
>turns or so to get around 33uH. That might not fit.
>Joop - pe1cqp

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