Receiver Sensitivity (and Transceiver Performance)

tweiss3

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Can someone explain, or point me in the right direction to understand sensitivity and how it relates to performance.

I've had an AT878uv for some time as my around the house HT. It has always had spotty coverage in the house at certain places, but I can get out at times pretty well. It has bothered me, but I usually use it DMR in the house, or analog in the yard without performance issues. If I'm actually on the radio, I'll just be in my office on my mobiles/base station, so it hasn't been a big deal.

I just picked up a FT-817nd, and started playing around with it yesterday. I noticed immediately that I get much better reception throughout the ENTIRE house. I can also get out to repeaters much more clearly on only 2.5W vs the 8W from the 878. So I decided to compare specifications (I also looked up the TH-D74a for fun).
FM WB sensitivity
AT878uv: 0.25µV
FT817nd: 0.20µV
TH-D74a: 0.18µV

Can someone explain how this relates to the better performance (other than its a CCR)? How does this translate to x-times better receiving. Does this have anything to relate to better transmission as well?

This makes me want to get a TH-D74a and/or another FT-817/818 for around the house use. I cannot believe the night an day difference, and that's only use the 2m/70cm/6m whip antenna on the front BNC.
 

prcguy

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Receiver sensitivity is actually one of the least important specs and is almost meaningless these days. All modern radios measure with more than adequate sensitivity and the ability to not overload and create internal IMD is much more important.

I've measured a Baofeng UV-5R and an Anytone dual band and a Yaesu VX-8R on a service monitor and the Baofeng came out the most sensitive. But this was on a test bench with only one signal being injected. Taking all three hand helds to a local hill top in Los Angeles, the Baofeng could not hear a reasonably strong signal on 146.52 simplex that the other two radios could hear clearly. I could easily talk to the station on 146.52 with the Baofeng but had to use another radio to hear them. The hill I was on is notorious for having lots of high level signals from local repeaters which can cause problems with lower quality radios.

So forget sensitivity and look at adjacent channel rejection or other receiver specs that will allow it to survive outside a test bench.
 
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chief21

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As mentioned, the design of the 878 is based on SDR technology, where the 817 is likely a traditional superhet design. Generally, the less expensive radios do not incorporate the same types or numbers of filters and other enhancements (like better selectivity) that often improve reception. The less expensive SDR models are also much more likely to be negatively affected by "unintentional signals", such as broadband interference from computers. monitors, routers, wall-warts and other devices.
 

tweiss3

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Receiver sensitivity is actually one of the least important specs and is almost meaningless these days. All modern radios measure with more than adequate sensitivity and the ability to not overload and create internal IMD is much more important.

I've measured a Baofeng UV-5R and an Anytone dual band and a Yaesu VX-8R on a service monitor and the Baofeng cam out the most sensitive. But this was on a test bench with only one signal being injected. Taking all three hand helds to a local hill top in Los Angeles, the Baofeng could not hear a reasonably strong signal on 146.52 simplex that the other two radios could hear clearly. I could easily talk to the station on 146.52 with the Baofeng but had to use another radio to hear them. The hill I was on is notorious for having lots of high level signals from local repeaters which can cause problems with lower quality radios.

So forget sensitivity and look at adjacent channel rejection or other receiver specs that will allow it to survive outside a test bench.
Ok, so funny you say that. First spec numbers (claimed numbers):
AT878uv: Adjacent Channel Selectivity=70dB, Blocking 84dB
FT817nd: Image Rejection=60dB, IF Rejection=60dB
TH-D74a: Spurious Rejection=50dB, IF Rejection=60dB

By claims, the 878 should be better, but its certainly not. Is is just that radio on chip is that crummy?

I consistently get the choppy audio, which appears to be overload. Also, my transmission is said to be much clearer and better received, even on 1/4 the power. I've tried the 701&771 antennas. Maybe I need to go back to the stock rubber ducky to "desense" the radio and eliminate overload. I think I need to replace this 878 in my daily pack.
 

prcguy

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That is curious but the 878 might be a direct conversion SDR where it only has a wide front end filter and the remaining filtering is done in DSP diddling one and zeros. In this case strong out of band signals can wreak havoc and overload the radio in general. The FT-817 is an older superhet where there is not only a wide front end filter but also a narrow IF filter that keeps out a lot of stuff. Not sure about the D74.

Ok, so funny you say that. First spec numbers (claimed numbers):
AT878uv: Adjacent Channel Selectivity=70dB, Blocking 84dB
FT817nd: Image Rejection=60dB, IF Rejection=60dB
TH-D74a: Spurious Rejection=50dB, IF Rejection=60dB

By claims, the 878 should be better, but its certainly not. Is is just that radio on chip is that crummy?

I consistently get the choppy audio, which appears to be overload. Also, my transmission is said to be much clearer and better received, even on 1/4 the power. I've tried the 701&771 antennas. Maybe I need to go back to the stock rubber ducky to "desense" the radio and eliminate overload. I think I need to replace this 878 in my daily pack.
 

SuperG900

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I've got an AT-878 - My experience is that it is fantastic at reception. It really pulls in a signal. The only downside is that, it being an HT, is that its great reception might fool you into thinking you can adequately hit a repeater when you're really on that repeater's fringe.
 
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RFI-EMI-GUY

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If I win the lottery I am going to buy a huge TEM cell that covers UHF and perhaps VHF and use it to expose portable radios to a fixed power environment representing the "real world", through multiple in band and out of band carriers generated by a comb generator and power amp. And then measure relative sensitivities using a desired signal *. In this way the non linearities and weak filtering n those cheap radios can be quantitively measured.

* In the analog microwave world they had something called noise power ratio using notched white noise and a desired signal in the notch.

TEM CELL
 
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RFI-EMI-GUY

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One other thing that is clearly a problem with scanners and liklely a problem with cheap chinese radios is that he internal electronics spray out noise and harmonics outside the plastic case. So you have an antenna 1-2 inches from a CPU inside and the counterpoise is the PC board the CPU is glued to. So the radio might work great on a signal generator with a hard wired 50 ohm connection, but it is drowning in its own noise with an antenna attached.
 

tweiss3

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Well, I tried the stock antenna to see if it would dumb the signal down enough to work properly and no. Now that I know there is better, I can't go back...........

It looks like I will be spending some more money in the future. Crap.
 
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