Levels measured in a Softrock TX.



I decided to record these measurements from a working Softrock. I actually used a v 6.1 but they all have similar circuits.

I used IQout http://www.m0kgk.co.uk/sdr/download.php to produce an IQ signal.
any SDR TX program may be used but this makes it easy to set.
These are my settings. I chose a fairly low frequency as many digital meters on the AC range will work at audio frequencies. 
Select your soundcard and drivers in use.
Connect the Softrock TX input to the Line Out.
Connect a power meter and dummy load to the Softrock output.
Power up the Softrock and USB.  GFGSR may be used to set the frequency to the lowest range covered. The latest version http://home.ict.nl/~fredkrom/pe0fko/CFGSR/ has buttons to make band changes quick. AND on the "Test" Tab it has a PTT button!

The Ensemble does not have COM port PTT so  you can use a cliplead to ground the top left-hand lead of U4 (Pin 4), activating PTT. Thanks for the pictures Robby! BUT see CFGSR above! http://www.wb5rvz.com/sdr/ensemble/index.htm

TOOLS:- The AC range of many modern digital meters is specified to work at audio frequencies. If you have one of these then it will probably tell you the approximate audio  RMS voltage present.

For the RF measurements this can not be used. An oscilloscope or just a simple diode probe will give a good idea of the levels present. The oscilloscope and it's probes will need to be specified to work on the test frequency. A diode probe should cover any HF frequency.
Diode probe example http://www.n5ese.com/rfprobe2.htm  (Lots of things on this site!) This will read RMS with a 10M Ohm digital meter. Multiply by 4 if you have a 1M Ohm meter.
Below, my build of one of these.

A piece of copper clad board cut to fit inside a pen shell. Lands made with small saw blade.

Components soldered. A stainless pin or needle may be difficult to tin with solder. A suitable flux may be needed.

Assembled. Not seen is the ground wire and clip that goes to ground on the item under test.
This probe actually has slightly different components and seemed to work better.
It is a single diode probe with a low-value resistor, not critical, 15K used here. It will read PEAK voltage.
A complete unit. This one is an integrating probe, a series 47K resistor followed by a 0.1uF to ground. The red clip is the ground connector, I'd run out of black sleeves! This may be used for measuring DC voltages where RF is present, like on the Softrock divider and mixer.


Set IQout "Volume" to a low setting, start it and enable PTT. Gradually increase this level until 500mW (1/2 watt) output is achieved. 500mW is 5V0 RMS, 7V1 peak measured at the antenna terminal with the dummy load in place. If this is not reached I hope the measurements will give a clue where to look! Note:- Do not exceed an audio input level of 0.7 volts RMS.
Set the audio to 0.5 volts RMS and see where they vary greatly from mine, below.
I have taken the schematic from Robby's SDR site. http://www.wb5rvz.com/sdr/ The red dots show where the exposed "hairpin" of the resistors are.

Audio voltages may be measured at the hairpins of R26, 25, 28, and 27.
For 500mW out I measured about 1V2 peak-peak on an oscilloscope and about 0V5 RMS with a digital meter.
Voltages at pin 2 and at the audio inputs will be similar.
If the input voltage is lower than this then check the soundcard mixer settings.
Some cards will go much higher,
March 2015:- Note tests done by Warren, 9V1TD indicate that to conform with FCC spurious emission requirements this level should never be increased above 0.7 volts RMS. If 1/2 to 1 watt output is not obtained then it is likely there is something wrong. For the 15/12/10 metre versions more drive may be used, the spurious signal is in fact lower at higher frequencies.


With an oscilloscope:- Voltages  PEAK TO PEAK. Make sure the oscilloscope and probes are specified for the frequency used. Frequencies higher than this may be used but sensitivity will be less and readings lower.
Normally a x10 probe will be used to obtain the specified frequency response. Check your measurements using the scopes "Calibrate" signal or put it to DC input and check the 5V line or a battery to be sure you are reading correctly.

Transformers will show a reading on three of the six connections, at one end of each winding..

One of my diode probes measures approximate PEAK voltage the other RMS. [RMS probe 0.7 times lower voltage. These RMS voltages were confirmed using a Bradley RF voltmeter with an unterminated probe] I found my RMS diode probe gave unreliable results at T2 where voltages are low.
Tests at 3.5MHz.
 Test Point Oscilloscope Peak to Peak Probe PEAK Reading Probe RMS Reading
 Each End of T2 Primary. (R30,31) 0V4 0V30V2 
 T2 Secondary (C20/L1) 0V60V30V3 
 Junction of L1 and C21 1V00V5 0V4 
 Q6 Emitter and Base 0V80V50V4 
 Q6 Collector 4V02V6 1V9 
 T3 Primary (R44 hairpin, same as Q6 collector) 4V02V41V9 
 Each end of T3 Secondary. (Q7,8 Gate) 2V00V6 0V7 
 Each End of T4 Primary (Q7,8 Drain) 6V5 4V1 3V0 
 T4 Secondary 8V05V1 
 Antenna (With 50 Ohm Load) 6V53V6 

These points are easily identified underneath the board, not so easy from the top but at least some of them are accessible. You do not need ALL these measurements. 
All measurements need a fine probe and care must be taken not to slip and short two points together.
NOTE Some of these readings do not seem consistent. Take them as approximate. The important thing is that they should increase as you progress through the transmitter.

I did some tests on my Softrock 6.3, which is really the same as an Ensemble but with plug-in coils. This seems to show that the Softrock TX is limited on the high bands.
These are derived from the amount of drive required using WSPR. This has a drive control calibrated in dB which seems reasonably close. (I measured -29db over it's range of -30dB, most likely the 1dB difference is my error.)
Using a fixed audio level I recorded the following WSPR attenuator values.

 1 Watt out on 80/40m -11dB
 1 Watt out on 15m -8dB This was the 1dB compression point
 1 Watt out on 12m -5dB
 1 Watt out on 10m -1dB The 1dB compression point was at 500mW so 1 watt required an extra 8dB.

I measured at the equivalent points below. Again the measurements from the probe, probably a different one than used above, do not match the scope readings at low voltages. 
Test at 28MHz.
 Test Point Oscilloscope Peak to Peak Probe PEAK Reading
 Each End of T2 Primary. (R30,31) 0V70V6
 T2 Secondary (C20/L1) 1V41V6
 Junction of L1 and C21 

 Q6 Emitter and Base 2V01V6
 Q6 Collector 5V02V8
 T3 Primary (R44 hairpin, same as Q6 collector) 

 Each end of T3 Secondary. (Q7,8 Gate) 3V01V4 
 Each End of T4 Primary (Q7,8 Drain) 8V04V9
 T4 Secondary 11V05V4
 Antenna (With 50 Ohm Load) 12V06V3

Driven to about 500mW on 28MHz. The audio drive was 1V8 peak to peak on the scope. This is only just below the 0.7 volts RMS maximum allowable figure.

You will realise that a diode probe is ideal for setting low TX powers. One may be made with the addition of a 50 Ohm load with a coax lead for the Softrock output. The dummy load may just be a 1/2 watt 50 Ohm resistor or a parallel combination. Even 4 off 220 Ohms and a 560 Ohm in parallel would suffice.

My method of checking the 1dB compression point.
This is a test to determine the maximum linear  output of an amplifier. An oscilloscope will not show small levels of distortion.
I measure the RF output across a 50 Ohm load reasonably accurately. I use an old Bradley RF meter, convert to dB.
A diode detector, like the probe above should measure PEAK RF voltage with a 10M Ohm meter. But check this. Calibration may still be necessary. 
There are lots of conversions two here http://www.tecmag.com/pdf/dbm_v.pdf Peak V = dB = mW
http://www.eeweb.com/toolbox/rf-unit-converter  Enter a figure, the rest is calculated. 
Using something like IQGen (see above) I increase the drive dB by dB. As power increases there will come a time when the output does not rise a whole dB. When the RF output lags the input by 1 dB then that's the 1dB point. 
This occurs at something like 1 watt on some low bands on some of my RXTXs but not that much in the mid-high bands.
Then a scope may be used to check the level. Do not exceed this level for linear modes like SSB, PSK.

Warren's way of checking linearity with a second Softrock http://groups.yahoo.com/group/softrock40/message/70260