Change in apparent sensitivity with Panadapter zoom

Bernie

I am using a 6700 with SmartSDR 1.3.0 on Windows 7.  I have noticed an apparent increase in sensitivity on 6-meters when the Panadapter zoom causes it to switch to Wide mode.  The RX preamp is set to +30 dB.  The AGC is set to Med and the AGC-T is set to 50.  I have set up two panadapters...one without any slice receiver (created by deleting a slice receiver), and one with a slice receiver tuned to 50.200 MHz.  The zoom in the panadapter with the slice receiver is greater so that I can see the envelope of a SSB signal.  Both panadapters are set to receive on ANT1.

The S-meter reading doesn't change (as noted in the slice receiver and DDUtil), but when I change the zoom in the panadapter without a slice receiver to change to "Wide," I can see an increase in amplitude in a USB signal in the Slice Receiver in the second panadapter.  The increase in amplitude is also noticeable in the audio output of the SSB signal.

The manual says in Section 7.3 that when the panadapter is in Wideband "the radio will open up the Bandpass Filters as wide as they can go..."  The RX and TX settings for bandwidth don't change, so my question is "Do I have control of the Bandpass Filters that change when switching to Wideband?" 

I have done a quick check on 20-meters and I do not see the same phenomenon.

-73-
Bernie Huth
W4BGH 

Tim - W4TME

Bernie - can you post screen shots of the the wide and non-wide panadapters on 6m so I can see what you are describing?

Bernie

I am posting two screen shots.  These show slice receivers in both panadapters, but I didn't observe any different behavior with or without a slice receiver in the upper panadapter.  I eventually shut down the slice receiver in the upper panadapter since I was concerned the effect might have been related to the fact both slices were tuned to the same frequency.  I believe the only difference was changing the magnification in the upper panadapter.  Also I tried to grab screen shots of the same station, but they are obviously taken at different times.  The AGC-T in these pictures was set to 30.





-73-
Bernie Huth
W4BGH

Bernie

I recently hooked a Fluke 6060A/AN signal generator up to my 6700.  The frequency was set to 50.200 MHz and the amplitude was -72 dBm (no longer traceable to the Bureau).  I grabbed two screen shots which I am including below.  I was using ANT2 (for convenience) with approximately 12-feet of RG8X connecting the signal generator to the 6700.  You can see from the pictures that in the normal mode (not Wide) with the RX preamp at +30 dB, the S-meter reads -73.9 dBm.  With the panadaptor in the Wide mode the S-meter reads -75.2 dBm.  Thus in the Wide mode the signal is LESS than in the "normal" mode.  This is in the OPPOSITE sense from my original report...so I throw my hands up and say I must be imagining things!!!





-73-
Bernie
W4BGH

Steve-N5AC

There are a number of things that can affect what you observe as the sensitivity if a receiver. In the specific case where you are switching between wide and normal mode, you are inserting and bypassing a 6m preselector. Preselectors have excellent out-of-band rejection, but because they are not perfect, there is in band insertion loss. This is most likely what you are observing in your test. But, sensitivity on 6m of the base receiver is really not important. All amateur radio transceivers have sensitivity that is generally considered insufficient for successful operations on 6m. This is why you should always use the preamp on 6m. The preamp successfully overcomes the noise figure of the receiver and any losses in the receiver including filter insertion losses. MDS with the preamp off on 6m is not a particularly interesting number -- it's always going to tell you to add some gain with a lower mouse figure amplifier to overcome the receiver noise figure.