Causes of IF feedthru
On 11 Jun, 22:52, "Joel Koltner" wrote:
Hi Dave,
Thanks for the information; it's quite helpful!
I was (am) committing the faux pas of feeding the mixer output directly into a
narrow bandpass filter...
For example, the popular
Mini-Circuits SBL-1 mixer seems to have LO-to-IF isolations of
anywhere from better than 65 dB (at HF) down to around 30 dB (at UHF).
If you're trying to tune a weak signal (say, 80 or 90 dB weaker than
the LO signal) then the residual LO feedthrough can cause the sort of
swamping you're seeing.
So... say I'm using a 45MHz IF, trying to tune 414MHz using the SBL-1 and an
LO of 414-45=369MHz (low-side injection) at the SBL-1's recommended +7dBm.
Presumably I'll see a 7dBm - 30dB (LO-IF isolation) = -23dBm signal at the IF,
but it should still be at 369MHz, right? *How does the LO "bleed through" to
the 45MHz IF... and what power level should I expect to see there?
---Joel
DBMs should be terminated in 50 ohms over a wide bandwidth. MCL has
recommendations concerning this.
Leon
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