By Walter Cates WD0GOF (UPDATE 10/30/2013)
36
3-11. SR-150 FILTER PASS BAND EMAIL DISCUSSION
(March 2012, a technical discussion between WDØGOF and K9AXN)
Hi Jim,
I have been playing around with the filter and balanced modulator in the SR-150. The specs on the SR-150 state
that SSB transmit audio response is 2100 Hz wide. That is, the -3db voltage points are 600 Hz and 2700 Hz
respectively.
The LSB osc freq is 1648.1 KHz; when the balanced modulator is driven with 600 Hz – 2700 Hz the double
sideband suppressed carrier would be as follows:
1648.7 KHz to 1650.8 KHz
and 1647.5 KHz to 1645.4 KHz
The USB osc freq is 1651.7 the balanced modulator will produce the following;
1651.1 KHz to 1649.0 KHz
and 1652.3 KHz to 1654.4 KHz
The blue band spreads are the wanted signals. The other sideband is out of the pass-band of the filter.
It seems to me, in order to maintain the transmitter max 3db roll off on both USB and LSB the filter roll off
point at the filter low end needs to be 1648.7 KHz the low freq for LSB. The high end roll off point is the high
freq for the USB or 1651.1. So this means the filter needs to be swept from 1648.7 to 1651.1 for a 2.4 KHz pass
band.
So this tells me that in LSB modulation at 600 Hz the power out should be no less than 3db down. When
modulated at 2700 Hz the power out should be full power.
In USB mod at 600 Hz it should be at full power and at 2700 Hz it will be no more than 3db down
Good analysis Walt. I believe the lopsided response is a result of moving the USB carrier crystal further into the
band pass so the CW/Tune signal will pass through the filter. I believe the filter has a 2.7Kc 3db pass band. The
USB carrier is about 350 cycles higher than the 3db point on filter skirt and the LSB is about 600 cycles lower
than the 3db point on the skirt. I may be wrong, it’s been a while but check that filter measurement.
Hallicrafters did this with the SR-150, SR-400, and SR-400A. The SR-2000 was fitted with a 30pf cap on the USB
crystal to drag it about 150 cycles into the pass band for Tune and CW enabling a bit more symmetry in SSB
mode.
Please let me know what you find --- been a while since I’ve been in that fine radio.
Summary of Contents for SR-150
Page 18: ...By Walter Cates WD0GOF UPDATE 10 30 2013 18...
Page 19: ...By Walter Cates WD0GOF UPDATE 10 30 2013 19 BEFORE CLEANING CLEAN CHASSIS...
Page 20: ...By Walter Cates WD0GOF UPDATE 10 30 2013 20 3 1 2 TUBE SOCKET CLEANING KIT...
Page 21: ...By Walter Cates WD0GOF UPDATE 10 30 2013 21 3 1 3 AIR VARIABLE CLEANING...
Page 30: ...By Walter Cates WD0GOF UPDATE 10 30 2013 30 3 5 5 K3 WIRING AND PIN OUT...
Page 38: ...By Walter Cates WD0GOF UPDATE 10 30 2013 38 4 DATA SHEETS...
