[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Gary Schafer]

Steve Thompson wrote:
> 
> Gary Schafer wrote:
> 
>>Good point Karl-arne.
>>Because of this it is important to keep the signal level relatively low 
>>into the receiver. 40 over S9 is probably pushing it on input level.
>>
>>An easy way to tell about receiver IM is to use two signal generators 
>>set at the same level a couple of Khz apart fed into the receiver. 
>>Measure the 3rd order product by comparing the difference between its 
>>level and one of the signal generator signals the same as described for 
>>measuring the transmitter.
>>This gives a two tone signal to the receiver with no IM products on them.
> 
> Don't take that for granted! Depending on the levels, good generators 
> with a good combiner should be ok, but I've seen people assume that and 
> then spend time chasing problems that were in the source signals, not 
> the amplifier.
> 
> Steve


Yes Steve, it is important. One signal generator coupling into another 
can create IM in the output amp of the signal generator. An attenuator 
should be used on each generator. Some generators are rather "dirty" to 
start with. Old tube type non synthesized generators are probably the 
cleanest unless getting into some rather expensive ones.

A TV type "splitter" can be used as a combiner. It should give about 25 
db of isolation on its own. Attenuators ahead of the splitter will give 
added isolation.

All this sounds like a lot of extra work to go though but when trying to 
make measurements but in the ranges being discussed things get a little 
more tricky.

73
Gary  K4FMX