HQ-170A Receiver Test Results

[W1VD]

Receiver: HQ-170A

Band	MDS	Blocking DR	Two-tone DR
		(20 kHz)	(20 kHz)
80 meters	-139 dBm	108 dB	81 dB
40 meters	-139 dBm	81 dB	57 dB
20 meters	-138 dBm	81 dB	59 dB

For comparison, measurements on other boat anchor receivers can be found at

http://www.w1vd.com/BAreceivertest.html

A study of the schematic shows the likely reason performance drops off markedly on the higher bands. On 160 and 80 meters the receiver operates in dual conversion mode and V3 is operated as an amplifier. On 40 meters and above a third conversion to boost image rejection is performed in V3. Halli chose to do this with V3 operating as a converter stage.

In order to verify that this was the problem, a quick modification was tried. The 2.58 MHz oscillator was disabled and the V3 converter stage was set up as a mixer - very similar to the first mixer V2. An external HP-8640B / amplifier was used as the 2.58 MHz oscillator. Blocking and two-tone dynamic range improved to the tune of about 15 dB. This still didn't equal the 80 meter performance but at least got it into the ballpark.

Since this receiver was on loan (and in mint condition) no effort was made to optimize performance by adjusting stage gain, LO levels, etc. This was a quick test to verify the problem. Seems it should be possible to add an additional tube that functions as the 2.58 MHz oscillator and use V3 either as a straight through amplifier on 160 and 80 meters or as just a mixer on the higher bands. 

The same circuit idea was used in the 160, 170 and 180 series receivers so performance on those models may have similar characteristics.   

   

         

[Jerry-n5ugw]

So I'm corn-fussed.. What numbers are we looking for to determine best. High MDS and Lw DR two-tone??

[W1VD]

A little info on the test and typical levels for a 'good' receiver...

Minimum discernible signal (MDS) is the signal level required at the receiver input to produce an output that is 3 dB above the noise. Measurements are normally made in a cw bandwidth (400 - 600 Hz if available) with the aid of a calibrated audio voltmeter. The larger the negative number the weaker the signal and the better the receiver sensitivity. Since there is greater atmospheric noise at the lower frequencies the importance of this number changes with the frequency. For example, a receiver with an MDS of -123 dBm might be sensitive enough for the lower bands (160, 80 and 40 meters) with a reasonable sized antenna but would be considered deaf on the higher bands (20, 15 and 10 meters). A good all around number would be roughly -136 dBm or better.

Blocking dynamic range is a measure of receiver densitization by a strong signal outside the desired receiver passband. The receiver is tuned to a relatively weak signal (-110 dBm) and an interfering signal 20 kHz away is increased in signal strength until the desired signal is decreased by 1 dB as indicated on the calibrated audio voltmeter. The level of the interfering signal is then referenced to the noise floor and this gives the blocking dynamic range - above the noise floor. The higher the number the better.  Good receivers typically test in the 110 dB range although excellent receivers can do much better.

Two tone dynamic range is a measure of receiver third order distortion. This occurs when two strong signals combine in vulnerable receiver stages and 'phantom' products are generated according to (2*f1-f2) and (2*f2-f1). To measure this, two signal generators are set for frequencies 20 kHz apart and are kept at the same output level. The generator levels are increased to the point where the third order products rise above the noise by 3 dB as indicated on the audio meter. The level of signal generator output is then referenced to the noise floor and this gives the two tone dynamic range - above the noise floor. Again, the higher the number the better. Good receivers typically test in the mid to high 80 dB range although excellent receivers test much better.

An interesting addition to any receiving setup is a front end step attenuator. Especially on the low bands, most receivers have sensitivity that is much better than required. One can typically 'click in' 10 or 20 dB of attenuation and still hear the band 'background' noise. While this doesn't increase the receiver dynamic range it does shift it to higher power levels. With this technique even a poor receiver can be made to perform admirably. The s meter will read low by the amount of attenuation so make sure to add that in when handing out signal reports!     

 

                     

[WA1GFZ]

A side note.
Around 1980 it was found through careful gain distribution the RF stage could be eliminated providing a jump in dynamic range and still give acceptable MDS.
Harris and Racal did this with success and moved the 2 tone dynamic range into the 90s with an MDS around -130 dBM. Going further Racal went to stronger MMIC amps in their first IF pushing the dynamic range over 100 dB at the cost of a few more dB loss in MDS. The second mixer beefed up with a stronger local oscillator of 23 dBM.
This sickness hits a brick wall when you get to the filters because most filters have a problem with more than 10 dBM signal. Above that the internal crystals can be damaged. So the ultimate machine has the least amount of gain ahead of the filters maintaining MDS. -130 dBM seems acceptable with a good 10 meter antenna.
The 90s come along with very high level mixers and Norton amps to take care of the dynamic range and MDS problems. There are a couple Italians who have h mode mixer front ends with homebrew roofing filters showing dynamic ranges around 120 dB.
Then new problem jumps out at you...close in phase noise.
As the sickness goes, HPSDR Mercury will hit the street soon which should jump slightly ahead of QSR1. I can't wait to measure the performance to see for myself an A/D eliminating the synthesizer and DDS problems. I have received a couple screen plots of Mercury phase noise performance that are shocking and need to be verified. 
It will be all about software if this is true