-10dBm RF Power Reference source-Output filter needs improving. - Page 1
Forum section: electronics / rf-microwave

author: Xnke
author_rank: Regular Contributor
timestamp: January 30, 2019, 06:00:57 am
content: Today I put together a little RF power reference for my home shack, and after getting it assembled I took it into the engineering lab my cousin works in, and we ran it through a few tests. He doesn't do RF work other than compliance testing, and they don't have the "proper" equipment to do the tests the way I know how to set them up, so we did what we could and got what we got.In the schematic above, you see I'm running a 3.3v CMOS clock oscillator, using a capacitor to remove the DC offset, then clipping with a pair of antiparallel diodes. This sets the RF peak voltage to +/-0.6V. After that, the clipped signal is fed into the 25Mhz low pass filter, then a 1.75db 50ohm pad, then the SMA connector.Output level is almost perfect, 25Mhz -10.78dBm as measured on the recently calibrated Rhode&Schwartz SA in the lab. The target was -10dBm, so I only undershot the pad a tiny bit. However, there is a spur at 75Mhz that the output filter didn't knock down enough-it's still -82dBm. It's *there*, but it's low enough not to be a problem in most cases.Is this just a case of needing another section in the filter to kill that off, or have I done something else that is upsetting the output filter? It's not *really* a big enough issue to worry about, especially if the 75mhz spur doesn't fluctuate in power after I get this thing into the aluminum case, as I'm only really concerned with *total* power output. If it turns out that I have -10.78dBm at 25mhz and -82dBm at 75Mhz, and they're both stable, then the reference is still fine. Actually, it would be OK anyway-as long as I know the total power-which is gonna be pretty slightly more than 0.100000005W total. That's pretty close, but since I'll be calibrating a wattmeter that goes down to -70dBm, it's probably important to get all the digits I can in there.Should I rebuild the whole thing and kill that spur at 75Mhz, or just not worry about it and call it good enough for a bench reference?

author: Xnke
author_rank: Regular Contributor
timestamp: January 30, 2019, 08:38:46 am
content: Tweaking the inductors up to 540nH and adjusting the first and last capacitive elements to 120pF and the filter straightens up a bit, forming a bit of a bandpass response and dropping the 50Mhz and 75Mhz transmission by another 15dB, while doing almost nothing to the desired 25Mhz signal.

author: David Hess
author_rank: Super Contributor
timestamp: January 30, 2019, 11:17:29 am
content: Harmonic filters often include a notch for the 3rd and 2nd if produced.I would not worry about the low level harmonic if its amplitude is on the order of the error from the temperature coefficient of the diodes. Leveled RF sources usually use a temperature compensated detector to drive an AGC loop to avoid this source of error.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2019, 12:45:16 pm
content: And, you can adjust the notch(es) by putting a little capacitance in parallel with the inductor(s). The lowpass profile changes slightly because of this, and is easily retuned.Tim