Transmit frequency response not flat in DIGU

Richard Berry

My output power varies from 20W to 30W between audio frequency of 200 and 2400 in DIGU mode running PSK31.  It is lower in the midrange frequencies between 1000 and 2000.  I verified that the TX equalizer is not used in digital modes ... got exactly same results with EQ OFF.  Anyone know why it's not flat and if there is any correction capability?  

Mike NN9DD

I think that is fairly normal behavior. The pitch of the digital signals varies in frequency and that causes rises and falls in power out. As you get to the edges it will generally decrease with all sound card type modes

Mike
N9DFD

Jim Gilliam

If the power increases with frequency sounds like the reactance of a coupling capacitor goes down as frequency increases.

George KF2T

Check the transmit band pass - not the EQ. It's in the PHNE tab on the SSDR display. Check the low cut and high cut values,

Walt - KZ1F

Are they used in dig[u|l]? I thought all that stuff was ignored when in dig mode.

George KF2T

Bandpass is used, but not the EQ or processor.

One thing I didn't suggest earlier is that the sound card may have some EQ in it. Check Windows settings and remove all "enhancements," too.

Richard Berry

SmartSDR says the TX EQ is not used in the digital modes but I tried it just to make sure, and it made no difference.  The TX bandpass is 100-2900.  Strangely, the dip in power is through the midrange frequencies, right where you would typically operate.  I'm not going to worry about it anymore, got more pressing issues at the moment.  

Bill W2PKY

One easy thought, watch the DAX level indicator when sweeping the frequencies to see that the input to the radio is flat.

Cal N3CAL

George,

What should one set TX band pass for using the DIGU Mode?   (low:0   High:5000)
Say for JT65/JT9 with a Rx bandwidth of 5000.... 

Cal/N3CAL

Bill W2PKY

0-4000 as rtty starts 4KC above the JT digital subband

George KF2T

Cal, I think the best answer is, "it depends." In a DAX or wideband sound card environment, WSJT can handle 4-5 kHz easily. If you need/want to tune that wide, it's kind of neat. I often look at 5 kHz. Not all apps support that width, so a narrower setting may be in order with them. Either way, it helps if there is not a crazy loud signal in the passband while you're trying to decode a weak one. Usually, good adjustment of the AGC-T prevents swamping, but having the offending signal out past the brick wall is always nice.

Richard Berry

Bill, good idea to watch the DAX level, so I did and it remains fairly constant while power varies from 40W in low range 400-1000 Hz and drops to 30W from 1000 Hz on up.  But, power is very sensitive to small changes on the DAX slider so perhaps it's not as constant as it appears.  Anyway, I'll live with it.