filtering of LO signals
 

Hi,
In my dual conversion receiver, I have a 45.455MHz second LO and a 45 -
75MHz first LO.
First IF 45MHz second IF 455KHz
Both these feed double balanced diode 7dBm mixers.
Also I have not filtered these signals at all, and the first harmonic is
roughly -20dBc - -30dBc.

Is it imperitive that I filter these for my receiver to work?
If so by how much?

Best regards,
John.

filtering of LO signals
 

Hi John,

If by 'first harmonic' you actually mean the second harmonic of your LO
(the 'first harmonic' usually refers to the fundamental frequency), the
answer depends a lot on how well your input is filtered, and on what
you have after the mixers.

This sounds like a 0-30Mhz shortwave receiver. If your input does not
have a good low pass filter for example, you may find strong FM signals
getting into the first IF passband as a result of the 2nd LO harmonic.
Note that your mixer inherently responds at odd harmonics of the LO
(3rd,5th...) and filtering odd harmonics out at the LO port does
little, so a good low pass filter is necessary anyway.
The second mixer is probably not as critical. Others will probably add
some more information.

Regards,
Glenn AC7ZN

filtering of LO signals
 

I have been characterizing some of the new Mini-Circuits LAVI-XXX series
mixers. These devices have an IP3 in the +35 to +40dBm range. Using HP 8644
signal generators to characterize IP3, I was only able to achieve
+33dBm.......IP2 was only in the low 60s. Return loss at the RF and I.F.
ports was only measured at 12 to 13dB.
I spoke to Mr. Lu Chen at Mini-Circuits and he explained that all sources
must have all harmonics down by at least -65dBc, and that the 6th and 9th
harmonics of the LO can be especially troublesome in degrading the
performance.
Using a low-pass filter on the LO improved the return loss to 20dB, while
the IP2 improved to +72dBm and the IP3 improved to +40dBm. It is always good
to have a clean LO source. If your 2nd harmonic is below -25dBc and higher
order terms are below -40dBc, you will have pretty good performance from the
receiver. A low-pass filter ahead of the 1st mixer will help reject that 90
to 120MHz image band.

Pete

"MadEngineer" wrote in message
oups.com...
Hi John,

If by 'first harmonic' you actually mean the second harmonic of your LO
(the 'first harmonic' usually refers to the fundamental frequency), the
answer depends a lot on how well your input is filtered, and on what
you have after the mixers.

This sounds like a 0-30Mhz shortwave receiver. If your input does not
have a good low pass filter for example, you may find strong FM signals
getting into the first IF passband as a result of the 2nd LO harmonic.
Note that your mixer inherently responds at odd harmonics of the LO
(3rd,5th...) and filtering odd harmonics out at the LO port does
little, so a good low pass filter is necessary anyway.
The second mixer is probably not as critical. Others will probably add
some more information.

Regards,
Glenn AC7ZN