On Wed, Sep 5, 2018 at 07:45 PM, <
rcbuck@...> wrote:
I used a Si5351A in a 20/30/40 meter receiver I recently built. The crystal filter frequency was 9 MHz. So my LO frequencies were 16.0-16.3 MHz, 19.0-19.15 MHz, and 23.0-23.35 MHz. I used low pass filters on the LO output to knock out the harmonics. The 20
and 30 meter bands share a common LPF with a cutoff around 19.2 MHz. The BFO output has a 9 MHz bandpass filter to clean up the output. I used output 0 for the LO and output 2 for the BFO. I never thought to look for the 9 MHz signal in the LO output. Thanks
for discovering this problem and the simple solution for it.
Ray,
Shielding costs weight and money is no surprise, it also gets good results!? Using the 5351 as a single
oscillator is actually not bad it when you try to use it for but LO and CIO the fun begins.? Solution use
a buffer that presents a high impedance to the 5351.? Lower the circulating current at the outputs and
the undesired cross-talk goes down.
Band pass filter can limit the impact of this but buffering first really helps.? I tried a 74LVT04 and the
spur was near gone (down in the HP8568B SA noise).? Simple fix as those are available as one
inverter per sot package.? They are cheap too Mouser gets 37 cents for them for qant=1.
So buffer then band pass you get even better result.
Me I cut my teeth doing superhet radios starting in the late 60s and the first thing to overcome was
unstable VFOs.? By time I started college I'd developed a deep understanding of PLL systems and
had made one for a radio using TTL/CMOS and whatever was available in 1971.? So many issues
of synthesized RF sources are seriously old hat.
What shocked me is after doing research and experiments I looked at many designs and noted
very few people understood this or paid attention to it.? Interesting thing is the only ones that?
do not have issues either use the Sa612 (high input impedance) and the QSD mixers (cmos switch).
Allison
