|Hey Gary, sorry for the dumb question. By “flat frequency response”, are you saying that it’s a flat attenuation as well as no distortion of the sample across the HF freqs?|
Thanks, good tip!
Reply to a comment by : NA6O on 2019-10-31
Good job, Scott... a very useful addition to any dummy load!
But you should add frequency compensation. Stray capacitance in parallel with the 56k will reduce the attenuation as freq increases. In fact, it may only be around 40 dB at 30 MHz. What you want is R1C1 = R2C2 where R1 is 56k, R2 is 56, and C1 and C2 are their respective parallel capacitance. As a starting guess, we can estimate that there might be C1 = 1 pF. Then what you need is C2 = 1000 pF in parallel with the 56 ohm resistor. Freq response will then be roughly flat thru the HF bands. This is exactly how compensation works on your 10X 'scope probe.
You can measure this. If you have only a 'scope for measurements, run your xmtr at constant power on 80m then on 10m. See if the output is roughly the same in both cases. If not, adjust C2. If you happen to own a VNA, or a spectrum analyzer with a tracking generator, then you can directly measure the overall freq response and optimize it very accurately. Then you will also know the exact attenuation factor, too.