Hi Roger
I have an Ensemble RXII and I have done measurements in the past that indicated to me that it's sensitivity was pretty uniform across the 160 - 10 meter range. I just made a quick check with a signal generator calibrated at -73 dBm using HDSDR, where I set this level for S9 on 20 meters. Here is what I measured:
1.840
??? -73
3.540??? -73.5
7.040??? -73
10.140??? -72
14.040??? -73 (calibration point)
18.240??? -71
21.040??? -70
24.040??? -71
28.040??? -74
I have long since removed the attenuator from the two low ranges just so I could use the RXII for such measurements. As you can see, the sensitivity is within 2 dB across this range. This is a standard build with no heroics other than removal of the attenuators.
I don't think you are going to find a substantial fault in the design that contributes to insensitivity. It is much more likely that the title of your post is correct and there is a soldering issue somewhere. :-)
The MFJ is a great diagnostic tool. It's output is a little rich and will overload the receiver (but will not hurt it) if you connect it directly. But if you can put a 20 dB pad in the ouput then, as you tune across the bands, you should see pretty close to the same levels. It is a great tool for the RXII to check as you cross band boundaries and be sure your filters are ok.
Hope that helps. Let us know how you make out.
Warren Allgyer
9V1TD
---In softrock40@..., <tharlam@...> wrote :
The sensitivity of my SoftRock Ensemble II is not adequate on 12 meters. This unit was purchased assembled and tested.
On 12 mtrs, on a good day, the band noise on my yagi is very low, but (just) above the sensitivity floor of my ICOM 738. The band noise is below the floor of the SoftRock however.
I checked the design of the bandpass filters to see if that might be the problem. According to a SPICE network analysis, the passband of the 8-16 mHz and 16-30 mHz filters is perfect for 50 ohm source and load. Thus, as long as the component values on the board accord with those shown on the schematic parts list, and if the cap Q is adequate at these frequencies, the filters do not appear to be the problem.
Next I checked the input SWR of the SoftRock. While I do not have a lab grade analyzer, I connected my MFJ 259B analyzer to the input of the SoftRock. For all the following tests, the local oscillator frequency was about 10 kHz above the test frequency. Measured input impedances were:
1.810 mHz: R=37, X=8, SWR=1.4.
??? At this frequency the bandpass filter is preceded by a resistor attenuator,
??? so a good SWR is to be expected.
14.193 mHz: R=10, X=28, SWR=5.9
24.944 mHz: R=16, X=44, SWR=5.5
28.010 mHz: R=229, X=178, SWR=5.7.
Assuming that the filters are correct, then I infer from these measurements that there is a problem with the load presented to the filters.
Assuming resistive impedances (and that I did the calculation correctly), an SWR of 5.5:1 corresponds to a loss of about 10 dB over that of a perfectly matched load. This could well be the sensitivity difference I observed between the ICOM and the SoftRock.
Has anyone else done sensitivity or input impedance measurements of the Ensemble II rx?
Does anyone on this list have the expertise (I certainly do not) to analyze the sampling mixer circuit including the coupling transformer to determine what impedance this is presenting to the filter?
Thank you.
Roger, VE7VV
Warren Allgyer