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Old 21st Jul 2022, 11:15 pm   #119
regenfreak
Heptode
 
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
Default Re: 6-gang FM stereo tuner heads

Quote:
With my RF sig gens I find the best isolation is achieved if I exploit the attenuator hold/lock feature. It's often possible to get an extra 10dB isolation per sig gen for free using this method. It locks in an extra mechanical attenuation stage at the sig gen output and then I can turn up the internal level to compensate. This only works up to the limit of the ALC hold/lock system but it usually allows an extra 10dB of mechanical attenuation in the step attenuator. The ALC bandwidth can also be set to narrow instead of the usual tens of kHz of ALC bandwidth. This really minimises any artefacts generated by the ALC.

One of my sig gens has an electronic step attenuator rather than mechanical but it still seems to perform very well for stuff like this. To get the best isolation from the 6dB combiner I generally add a 10dB attenuator at the sum port of the combiner. This combiner typically has >46dB isolation across the HF and VHF bands if well terminated at the sum port.

I've got lots of other combiners here including a wideband resistive combiner that has about 19dB port isolation and 9.6dB through loss. This performs a bit better than the classic 6dB resistive splitter in terms of port isolation.
I would be interested to see the measurement of IIP3 and OIP3 of a ring mixer,e.g. ADE-1 or SBL-1 since you have the right equipment to do it. It will need +7dbm signal to drive the LO oscillator. Here is a rather long-winded article about IIP3 and OIP3 measurements:

https://www.markimicrowave.com/blog/...input%20signal.

I dont think the duplex low pass filters are necessary for quick and simple measurements.


I have built a 5-pole Chebyshev 10,7MHz IF filter and have realised that the ferrite cores of my inductors are super lossy at 10.7MHz. This is the same inductors i used for my LC impedance matching pads used in the measurements of the single and double ceramic IF filters (with CA3053 buffer amp). This explains why I got so much insertion losses in the mesurements. I really have to use the proper toroids suitable for 10.7MHz . I was lazy and used whatever I got in my junk box.

Last edited by regenfreak; 21st Jul 2022 at 11:32 pm.
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