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SX-28A Receiver Dynamic Range Test Results




 
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Author Topic: SX-28A Receiver Dynamic Range Test Results  (Read 29752 times)
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Steve - WB3HUZ
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« Reply #25 on: October 25, 2008, 03:33:30 PM »

Or in other words,


NFn = NF1 + (NF2 1)/Gain1 + (NF3 1)/(Gain1 Gain2) + .. +  (NFn 1)/(Gain1 Gain2 .. Gainn-1)

where item 1 is the RF amp, 2 is the mixer, and so on.


I-f filter loss, unless grossly high, won't have an affect on MDS in receivers that have a reasonable amount of gain ahead of the filters - an rf amplifier and mixer for example.  15 - 20 dB of 'front end' gain will make 10 dB loss that follows invisible as far as noise figure or MDS is concerned. 

An unusual receiver like the Squires SS-1R, with only a single relatively low gain active stage (on 40 meters), would be more likely to suffer MDS degradation with high filter loss.   
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KM1H
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« Reply #26 on: October 26, 2008, 08:25:06 PM »

This is where a 6 or 8 pole AM xtal filter would help. Mount under the chassis. There is more than enough IF gain in a SX-28 to overcome the insertion loss.

The single pole filter has poor skirt selectivity as shown in the manual. Turning off the AVC and backing down the RF gain may help a little.

Carl
KM1H

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Steve - WB3HUZ
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« Reply #27 on: October 27, 2008, 12:34:31 PM »

Pre-WWII receiver are generally not give you the selectivity you require in this 5 kHz spacing scenario. More modern receivers will, but your audio quality will be reduced. With all the open space on 160, why in the world are AMers bunching so close together? Seems like operator problems rather than a receiver problem.
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wa2dtw
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« Reply #28 on: October 28, 2008, 09:50:15 AM »

Steve
I try to keep the BC610 parked on 1885, because it is difficult to accurately change frequency using the tuning unit.  (a slight touch can move you about 10kc).
Lately, there are is significant AM activity on 1900 and 1878.   A strong signal there will wipe out anything at 1885 on the SX28.

73
Steve WA2DTW
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Steve - WB3HUZ
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« Reply #29 on: October 28, 2008, 11:17:26 AM »

That's fine. What I've often found when I was in a QSO on 1885, someone else would start up a QSO on 1880. I find the sort of operating self defeating on AM. Spread out.
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WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG


WWW
« Reply #30 on: October 29, 2008, 06:43:54 PM »


There are some jim dandy ceramic filters that work great for 455kc IFs and also for 455KHz IFs as well... they're usually made by MuRata, although I think there are some from Toko Labs as well. Small and of good skirt for AM... some signal level padding going in may be required, and then some signal boosting going out depending on the levels in a given rig, but to go back up you can use a FET or bipolar (they'll handle the B+ if properly selected - come to think of it some MOSFETs will too) and the stage can be very simple (a triode config in solid state). Easy solution, and hides on a small board under the chassis, no blasting and reduction of the old rig's value...  Grin


                    _-_-bear
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_-_- bear WB2GCR                   http://www.bearlabs.com
Steve - WB3HUZ
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« Reply #31 on: October 29, 2008, 08:40:54 PM »

No need for all that. Just get the Kiwa units.


There are some jim dandy ceramic filters that work great for 455kc IFs and also for 455KHz IFs as well... they're usually made by MuRata, although I think there are some from Toko Labs as well. Small and of good skirt for AM... some signal level padding going in may be required, and then some signal boosting going out depending on the levels in a given rig, but to go back up you can use a FET or bipolar (they'll handle the B+ if properly selected - come to think of it some MOSFETs will too) and the stage can be very simple (a triode config in solid state). Easy solution, and hides on a small board under the chassis, no blasting and reduction of the old rig's value...  Grin


                    _-_-bear
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N3DRB The Derb
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« Reply #32 on: October 31, 2008, 03:53:59 AM »

Quote
NFn = NF1 + (NF2 1)/Gain1 + (NF3 1)/(Gain1 Gain2) + .. +  (NFn 1)/(Gain1 Gain2 .. Gain-1)

hey man......how'd you get so funky? I don't believe that's ever been done on a record before.  Tongue
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Steve - WB3HUZ
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« Reply #33 on: October 31, 2008, 09:36:55 AM »

It's gonna wipe out the moon walk!

Quote
NFn = NF1 + (NF2 – 1)/Gain1 + (NF3 – 1)/(Gain1 – Gain2) + ….. +  (NFn – 1)/(Gain1 – Gain2 ….. Gain-1)

hey man......how'd you get so funky? I don't believe that's ever been done on a record before.  Tongue
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