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QRP transmitter PA output and lowpass filter?

Nov 23rd 2014, 16:30

KA8SYX

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Hi Everyone -
How can I determine whether a certain PA transistor used in a QRP CW transmitter in combination with a particular lowpass filter design will need an output transformer to match the output impedance to a standard 50 ohm impedance? I have a transmitter I built from a kit a while back that ended with a simple pi-type (two caps/single inductor) filter and based on some reading I did was leery about it meeting spectral purity requirements. Furthermore, more reading I did on the transmitter design suggested that the lowpass filter was not well-suited to the PA in its stock configuration, with the parts supplied. So I read a piece on RF amplifiers by NA5N and used his equations to figure out a new pi-type filter. The stock filter made the PA transistor run very, very hot, which to me meant a poor match. Output was very low, too. The filter I devised, still a pi-type so I could use the existing PC board, used a hand-wound toroidal inductor and silver mica capacitors instead of the ceramic caps and solenoidal coil provided in the kit. The new filter, based on new observations, permitted cooler PA operation, higher output and it sounded better in a nearby receiver. However, it is still a three-element lowpass filter. But my attempts to build a filter with more poles keep falling short even when I use published designs. Where I am looking for an appropriate cut-off above 10.5 MHz (30M transmitter), I cannot get the impedance of the filters I build to depart from transparent (antenna analyzer right to filter input to 50 ohm dummy load attached right to the filter output) above about 9.9 MHz. Anyway, I'd like to be able to use this transmitter in regular operation, as the improved 3-pole pi output seemed to work well. Any advice? Thanks very much! 73, Ray KA8SYX
Nov 24th 2014, 21:06

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

If the filter cut off is too low, the most likely answer is that you are using toroidal inductors with too much inductance. It is quite common for folks to miscount the number of turns--each time the wire goes through the hole counts as one turn--one loop is actually two turns.

Zack Lau W1VT
ARRL Senior Lab Engineer
Feb 23rd 2015, 02:09

WB2IQG

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
A 5 order Chebyshev low pass filter designed in SVC Filter Designer 2.12 software (which I think is Elsie derived) has inductor and capacitor values which are really different than what is found in the "ARRL Handbook" for the same filter cutoff frequencies. I have often wondered which one is correct or are both correct with different return loss figures in the desired pass area of the filter which is an important filter performance criteria.

I have also wondered if the toroid mix should be optimized for the pass frequency or for the harmonic frequencies which are being rejected by the filter. T-xxx2 vs T-xxx-6 for the HF bands.

Thanks, Phil WB2IQG
Feb 23rd 2015, 15:39

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
If you are trying to match up Chebyshev designs, you need to match up the passband ripple in addition to the cutoff frequencies. As the amateur bands are narrow, it isn't unusual for designs to accept some passband ripple in order to use standard value parts--which typically makes the design less expensive to duplicate. You could also use two capacitors in parallel to obtain non-standard values.

Zack Lau W1VT
ARRL Senior Lab Engineer

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