Selecting class of amplifier for a multicarrier FM broadcast - Page 1
Forum section: electronics / rf-microwave

author: vu2tve
author_rank: Contributor
timestamp: January 25, 2022, 09:14:03 am
content: I would like some signal processing advice for a project. I'm working on an SDR based setup that produces multiple FM carriers in the FM broadcast band, and am wondering the class of operation of the RF PA.Now, a straight answer for this would be of-course class-A + low pass filter, but I am wondering if I can get away with class-AB and a tight bandpass filter.I did a couple of simulations in gnuradio to see the nature of harmonics being produced in class-ab, and while of course the class-a fft looks a lot cleaner, could I get away with class-ab + tight band pass filter.If anyone could offer practical advice, it would be very helpful. Class-A would mean loss in operating efficiency, but if the distortions in AB would be too much, then I'd have to go with class-A (or consider AB-push-pull)73 // VU2TVE

author: gf
author_rank: Super Contributor
timestamp: January 25, 2022, 09:46:29 am
content: In your AB simulation, you distort each signal individually and then add them up - I guess this is not what you really intend.Don't you want to add the N signals first, and then send them together to the (distorting) amplifier?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 25, 2022, 10:02:00 am
content: AB is fine if it's push-pull.With distortion, I don't see that an effective result will be had for multiple carriers at a time. If a diplexing filter can be had, this works with multiple class C amplifiers feeding in their respective channels; making such a filter, good enough to avoid crosstalk, is not trivial. And, I guess, not what your synth is expecting (single pre-mixed output?).Tim

author: vu2tve
author_rank: Contributor
timestamp: January 25, 2022, 10:05:28 am
content: You're right. I modified the flowgraph and here are the screenshots..Also realized that it is really class-B, not ABThere's a number of harmonics, but perhaps with a BPF it could work.I guess I could also test with actual signals and FM modulate them and demodulate and see what they sound like

author: vu2tve
author_rank: Contributor
timestamp: January 25, 2022, 10:08:18 am
content: Yeah. I want to avoid the class-C/diplexing filter. Single SDR output with multiple carriers to PA stage seems cleaner approach.

author: vu2tve
author_rank: Contributor
timestamp: January 29, 2022, 02:49:30 pm
content: Hi,I made a calculator to help out with some numbers as a google sheet at this link. This is publicly editable, but I will change that after a few days have passed.https://docs.google.com/spreadsheets/d/1H0mCZAM0MhHkbXDTtomCzDjg0lkKSL3_eBUJIuL1NSY/edit?usp=sharingI would really appreciate some feedback and whether I made any mistakes. The sequence of calculations is as follows:Desired output power per carrier (Editable) = 2WNumber of carriers (Editable) = 5Characteristic impedance = 50 OhmOutput rms voltage swing per carrier = Square_root(2W x 50Ohm) = 10VTotal output voltage swing = 10V x 5 (carriers) = 50VOutput power required of class-A amplifier; Pout = (VxV)/R = 50x50/50 = 50WMax. achievable efficiency of class-A amplifier = 50%DC power consumed by class-A amplifier ; P_tot = 50W / 50% = 100W = Power that will be dissipated by the heatsinkDrain supply voltage (Editable) = 50VRough estimate of Quiescent drain current = DC Power / Supply voltage = 100W / 50V = 2ALoad line resistance R_L = (VDD*VDD)/(2*Pout) = 25 OhmMax allowable Junction temperature (from Datasheet) = 175CCase to junction thermal resistance (from Datasheet) = 1.1 C/WMax allowable case temperature; T_case = 175 - (P_tot*1.1) = 175 - 110 = 65CAmbient temperature (Editable) T_ambient = 30CTarget thermal resistance of Heat-Sink = (T_case - T_ambient) / P_tot = (65C - 30C)/110W = 0.4 C/WNow 0.4C/W is probably difficult to realize, but these are the numbers for calculations' sake.Does this look correct