RSP1A Longwave sensitivity

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ON5HB
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Re: RSP1A Longwave sensitivity

Postby ON5HB » Sat Jan 05, 2019 4:00 pm

You should not have those high sensitivity numbers on de low HF/VLF bands as it will overload your receiver.
Most HF/VLF receivers have less sensitivity below 40m band as signals can be very strong and often are.
To reach those numbers you need to turn Pre-Amp on with the Icom7300, but if you do the overflow starts blinking all the time.

Those numbers of the RSPDuo will overload the receiver all the time.

I have compared the RSP1A with my Icom7300 on the same G5RV and 160m Inverted-L, the sensitivity and quality of the reception in about the same.
Most of the time I need to attenuate with 20dB to keep it out of overloading.

Also the shematics show that the Hi-Z port has no gain-control and it overloads pretty quickly, just read from the presentation PDF:

Unlike the RSP2, the Hi-Z port on the
RSPduo now incorporates both a MW notch filter and a 0-2 MHz low pass filter, which should
help reduce overload from strong MW AM signals and spurious mixing products from HF.
The
Hi-Z port should deliver the best performance for LF, MW operation and ought to be far
superior to that on the RSP2. The 50Ω ports on Tuner 1 and 2 deliver very good performance
from LF to 2GHz, but deliver the best performance for HF and above due to the extra
preselection filters which are not available on the Hi-Z port above 2MHz.
The 50Ω ports on Tuners 1 and 2 should deliver similar performance to the RSP1A, but with
improvements in both the MW and VHF FM notch filter rejection and also a reduction in
spurious mixing products from UHF appearing in Band 3 (120 – 250 MHz)


You are comparing apples and oranges, the Hi-Z is nice, but I wouldn't use it in areas with a lot of high powered stations or where there is a load of QRM.
I have seen these problems with RTL dongles that do direct-sampling on HF with no gain control, in the evening you can't do anything with them.
Nice to have but not very useful when using big antenna's.

Better use the normal ports with a balun to feed it from a open-line.

Just my 2 cent's on the matter. Happy new-year!
Last edited by ON5HB on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Greetings,

Bas - ON5HB

sdrom33
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Re: RSP1A Longwave sensitivity

Postby sdrom33 » Sat Jan 05, 2019 4:52 pm

Hi Arcosine what is your receiver the 1a or the duo?

Hi ON5HB, you mean I do not need such sensitivity & the 1a is O.k.? For me one input is O.k.

Thanks!
Last edited by sdrom33 on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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arcosine
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Re: RSP1A Longwave sensitivity

Postby arcosine » Sat Jan 05, 2019 5:33 pm

Hi

I have the the 1a.

I did a comparison with my Yeasu ft-540d, signals I can hear on the Yeasu are buried in the noise on the 1a, NDB 411 khz, BA.

Tony
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Tony
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Roger
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Re: RSP1A Longwave sensitivity

Postby Roger » Sun Jan 06, 2019 3:05 am

There has been a lively discussion on this subject so far. Several people have made comments on my posts and I want to clarify some issues.

Geographic location

Reception in Europe on frequencies below 30 MHz will be much different than what we experience in most places in North America (where I live). The population density is much higher in Europe and this results in higher signal levels and man-made noise on most of the bands. When I lived in Holland shortwave listeners used to call it "RF alley" and my poor Kenwood R-5000 would sometimes overload without the attenuator on. When I listen to Web receivers in Europe I see more activity on the amateur bands than I do here in the Pacific Northwest. MW stations (540 -1700 kHz.) and FM stations are also not much of a problem unless one lives near or in a big city. Rural areas and small towns are often many kilometers away from a city and don't have high power AM or FM stations nearby. In North America we don't have longwave radio stations like Radio Algeria on 252 kHz with transmission powers of 1500 kW during the day and 750 kW at night or other powerful LF stations like these >>
https://www.hfunderground.com/wiki/Longwave_Broadcast_Stations

The end result is that the band below 500 kHz does not have many strong signals for North American DXers to contend with. I live close to a major airport and the Non-Directional beacons are only transmitting with 25W. Most NDB's are under 150W and the strongest are 2 KW. This means that listeners can use a sensitive receiver and a reasonable antenna not be concerned about overload but only if MW stations do not cause spurious images below 500 kHz.

Spurious Signals

Serious VLF and LF NDB DXers use narrowband radios like the Icom R75. They are sensitive receivers with adjustable bandwith and minimal spurious from MW and other out-of-band signals. The RSP is a wideband receiver and this can result in overload and/or spurious signals if precautions are not taken. Use of Low IF and a suitable LO frequency is required because this will give fewer MW "ghosts" below 500 KHz. In areas with MW stations nearby an RF low pass filter and/or a MW rejection filter is required. A preselector will also bandpass filter the signal input into the RSP and allow more RF and IF gain and fewer spurious signals. Users with MW spurious or overload problems should read this SDRplay app note https://www.sdrplay.com/docs/SDRplay_Optimising_Performance_RSP1A_LF_MW_HF.pdf

Receiver Sensitivity

Several posters questioned whether I was able to use my RSPduo at maximum RF Gain. The answer is yes. However the antenna I used was not designed for LF so signals and noise were lower than one would get with a proper LF antenna. The screenshots posted earlier were taken at my location with RF Gain at maximum to get the best Noise Figure and highest sensitivity possible. The IF gain was at mid-position but this did not affect the NF to a large degree but it did lower the level into the AD in order to prevent overload.

For those living in areas with higher signal levels or better antennas they will have to reduce the RF gain considerably in order to avoid AD overload or reduce intermodulation. Each dB of attenuation increases the Noise Figure by 1 dB and reduces receiver sensitivity. In this situation the benefits of the HiZ port will not be realized and the RSP1A may be a more cost-effective solution.

Antenna Considerations
The selection of an antenna will have a considerable effect on the signal levels being received. Higher gain antennas will deliver more received signal and RF Gain will have to be reduced. An external attenuator can be used but I suggest that the RF Gain control be used, if possible, because that way the SDRuno S-meter will still be accurately indicating the receive level at the antenna terminals. Active loop antennas use an amplifier and not too much gain should be used with this type of antenna. The designer of Wellbrook antennas recommended that I not buy the Pro model because there is too much gain for the RSP.

Noise - Man made or atmospheric

Other posters have stated that noise levels are sometimes high enough that a high sensitivity receiver is not required. They are correct in this assertion. Summertime atmospheric conditions raise the noise level considerably on HF and lower frequencies and high S meter readings are common. In the winter months atmospheric noise levels are much lower but man-made noise is still a problem. We live in a digital world and the level of unwanted RFI gets worse with each passing year. Noise cancelling devices like the Timewave ANC-4 or MFJ-1026 can provide some relief but they need to be manually adjusted and can become tedious to operate.

Summary

The RSP family of products provide a low-cost way for hobbyists to have fun with a wideband Software Defined Radio (SDR). The selection of which SDR product to buy depends on the budget and the geographic location, antenna, local noise levels and band(s) of interest of the purchaser. These devices provides a capability that would have cost ten times as much only a few years ago. But to exploit all the benefits will require some experimentation and a thorough review of the documentation available.

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Roger
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Re: RSP1A Longwave sensitivity

Postby Roger » Sun Jan 06, 2019 4:14 am

Earlier I made some comments about Noise Figure and SNR in a receiver. Some members of this forum may not be familiar with these terms so I will attempt to explain what they mean and how they are used. I will try to give an overview and avoid being technically rigorous.

PART 1

What is the term Noise Figure (NF)? In electronic circuits noise will be generated by the random thermal motion of charge carriers (usually electrons). Even a simple resistor will have random noise present at the terminals. Other devices like amplifiers, mixer circuits, filters etc. will also generate noise. So if you put a weak signal into the input of a receiver the internal noise of the receiver will limit the reception quality of the signal. One way of comparing receivers is by calculating the Noise Figure (NF). This can be done by putting some noise, at a certain frequency, into the input of a receiver and measuring how much noise comes out of the receiver. Lets say we measure at 1 MHz. (my example in the previous post) and find that the noise coming out of the receiver has 100 times the noise power compared to what we put into the receiver. The ratio, called the noise factor (nf), is 100 in this case. The decibel (dB) is a useful expression in RF engineering and we if we take 10*log(nf) we get what is known as the Noise Figure, in dB, of the receiver. In this example 10*log(100) = 10*2 = 20 dB. The lower the noise figure (NF) the better the receiver in terms of thermal noise performance. If we take the measurement at different frequencies like 1, 20 or 100MHz we will get different NF calculations for the RSP. One can see that the NF is a useful tool to compare one receiver with another. . That is why I used it to compare the RSP1A with the other RSP devices. At 1 MHz. SDRplay specifies the Noise Figure of the RSP1A as 21.22 dB and the RSPduo HiZ port as 11.66 dB. So the RSPduo is a less "noisy" receiver at this frequency. However, the benefits of a lower NF and higher sensitivity will only occur if the atmospheric and man-made noise levels are not high. As other posters have pointed out noise on the LF, MW and HF bands can be quite high, especially during the summertime, and a reduction in RF gain or external attenuation will be required. In these cases the RSP1A and RSPduo will have the same level of performance.

Receivers with a lower NF will require better design techniques and higher quality components. So there is a trade-off between cost and performance. So what is a good Noise Figure for a receiver used by hobbyists? To a large extent that depends on the atmospheric/galactic noise, man-made noise, antenna characteristics and what type of signals are being received. What is required is a sufficient Signal-to-Noise Ratio (SNR) to meet the reception objective. For a voice signal a 12 dB SNR in a 1500 Hz. bandwidth gives reasonable quality audio. Some numbers help to understand how we can achieve this desired SNR. The noise power can be expressed in dBm which is a logarithmic way to measure power. It is referenced to 1 milliwatt of power with negative numbers being less than 1 milliwatt and positive numbers being more. If we terminate the 50 ohm input to a RSP with a 50 ohm resistor, instead of an antenna, it will generate thermal noise. At room temperature this is -174 dBm if the bandwidth (BW) is 1 Hz. In 1500 Hz there will be 1500 times the power or 10*log 1500 = 32 dB more noise. So in a 1500 Hz. BW the total resistor noise power at the RSP input is -174+32 = -142 dBm. From above we learned that the receiver adds internal noise and has a Noise Figure. A good quality receiver on the HF bands, like an RSP has a NF of around 18 dB so if we add this to -142 dB we get -124 dBm at the output of the receiver. This is often called the “noise floor” or Minimum Discernible Signal (MDS) of the receiver and this is a typical for a modern HF receiver with the bandwidth set to 1500 Hz. In order to receive a voice signal and demodulate it with a 12 dB SNR the received signal would have to be -124+12 = -112 dBm (0.56 microvolts) which would be around S2-S3 on the RSP S meter.

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Roger
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Re: RSP1A Longwave sensitivity

Postby Roger » Sun Jan 06, 2019 4:24 am

Part 2

When we hook up the receiver to an antenna we get a lot more noise that when we use a resistor. Received atmospheric noise (lightning storms) power levels will be much higher and it is not unusual with a good antenna to see noise up to -91 dBm (S6) or more on the lower MW or HF bands in the summertime. As we increase in frequency (beyond HF) atmospheric noise decreases and galactic noise becomes the dominant noise source but at much lower levels.

Man-made noise is also a real detriment to reception, especially on the HF bands below 30 MHz. Man-made noise can come from AC power lines, digital devices, Ethernet cabling, grow lights, TV’s and a host of other sources. Some users complain of S9 noise levels (-73 dBm). With atmospheric noise and man made noise at these higher levels the internal noise of the receiver is not a concern and the receiver Noise Figure can be much higher (25-35 dB) without affecting reception.

Beyond 30 MHz there is a desire to receive weak signals from satellites, low power devices and over long distance (line-of-sight) paths. So receiver designers use design techniques and components which reduce the NF so these weaker signals can be received. For example an RSP operating at 144 MHz. (the 2M ham band) has a NF of only 3.3 dB which results in very little internal receiver noise.

A number of hobbyists are interested in receiving weak signals below 500 kHz. The signals can be from amateur radio stations sending low power WSPR transmissions in the 2200M and 630M bands, from Non-Directional Beacons or distant longwave broadcast stations. Reception below 500 kHz can be very noisy during the summer due to lightning but in the winter the band is often fairly quiet and long distance reception is possible.

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arcosine
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Re: RSP1A Longwave sensitivity

Postby arcosine » Sun Jan 06, 2019 2:13 pm

Hi,

Do mention dynamic range.

I heard my first ham signal on 2200 meters yesterday, KD0VBR beacon, 900 miles away. Max power on 2200(137 khz) is one watt radiated. Not everyone can have a 900 ft wire antenna, its a half wave near 472 khz. The 1a needs an front end with some gain and selectivity. I am going to try adding a simple fet Q multiplier to my tuned matching transformer, I think the 1a can supply a DC bias thought the antenna port. The Q now is about 5 to 10, Id like to get 10 dB gain and a Q of 50 to 100. I have to get the low noise fet first.

Tony
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Tony
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Roger
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Re: RSP1A Longwave sensitivity

Postby Roger » Sun Jan 06, 2019 6:08 pm

ON5HB wrote:You should not have those high sensitivity numbers on de low HF/VLF bands as it will overload your receiver.
Most HF/VLF receivers have less sensitivity below 40m band as signals can be very strong and often are.
To reach those numbers you need to turn Pre-Amp on with the Icom7300, but if you do the overflow starts blinking all the time.

Those numbers of the RSPDuo will overload the receiver all the time.

I have compared the RSP1A with my Icom7300 on the same G5RV and 160m Inverted-L, the sensitivity and quality of the reception in about the same.
Most of the time I need to attenuate with 20dB to keep it out of overloading.


In general I agree with you when talking about the HF ham bands. With current solar conditions I do not see a lot of strong stations on most of the HF bands unless a contest is running. Maybe it is my location or my G5RV antenna. My comments have been directed at reception below 500 kHz and based on my observations here in North America during the winter. I do not have any longwave stations or strong MW stations nearby so overloading is not an issue for me. That is not the case for many listeners, especially in Europe, and they will have overload without turning off preamps, reducing gain or using attenuation.

ON5HB wrote:Also the shematics show that the Hi-Z port has no gain-control and it overloads pretty quickly,


The block diagrams published by SDRplay are not that detailed. The HiZ port in the RSP2 and RSPduo does have gain control with 36 dB of range. It is described in this SDRplay technical document https://www.sdrplay.com/wp-content/uploads/2018/06/RSPDuo-Technical-Information-R1P1.pdf


ON5HB wrote:]You are comparing apples and oranges, the Hi-Z is nice, but I wouldn't use it in areas with a lot of high powered stations or where there is a load of QRM.
I have seen these problems with RTL dongles that do direct-sampling on HF with no gain control, in the evening you can't do anything with them.
Nice to have but not very useful when using big antenna's.


I agree with you. When there are a lot of high-powered stations or QRM it does not matter if one uses the HiZ port or the 50 ohm ports. The RF gain control is reduced and/or attenuation is required. In this case it does not matter whether a RSP1A, RSP2 or RSPO duo is used - the results will be pretty much the same.

But this thread is about trying to receive weak stations below 500 kHz. I was a shortwave listener long before I became a ham and the objectives are much different. A serious SWL DXer will try everything they can to improve their receive station in order to hear distant stations. For LF DXing the use of low pass filters, MW notch filters, preselectors, noise cancellers and rotatable loop antennas it is possible to reduce the level of unwanted signals into the receiver. Listening in the wintertime means lower atmospheric noise levels. By doing the above it is possible to use sensitive receivers and that is the point of some of my comments.

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ON5HB
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Re: RSP1A Longwave sensitivity

Postby ON5HB » Mon Jan 07, 2019 5:35 pm

Hi Roger,

I agree with you, but an SDR receiver is a different puppy then a normal (old) receiver.
There is no need to feed it with high-levels, in fact when you do strange things may happen.

I did a test with the RSP1A on a G5RV and setting the band to 1-4 MHz in rate, all stations at 1 MHz worked fine, but in the 3-4 MHz range ghost signals started to appear filling the entire 80m band with AM-stations.
This will happens to any user as AM(MW) stations get very very very strong at night even when not close to you and setting the sample rate too high.

Very weak 0-500 KHz stations are a problem for SDR, to listen to such an old analogue receiver with much smaller band-pass-filters is far better.
One can not expect a 150 dollar box to match it, as the band-pass-filter is far too wide, sorry it is.
You could improve it by building your own very narrow filters and tune them to the freq you want to hear.

Listening to such specific low bands for weak signals is better done with old school stuff as the filtering is done BEFORE tuning and not after.
Last edited by ON5HB on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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Greetings,

Bas - ON5HB

glovisol
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Re: RSP1A Longwave sensitivity

Postby glovisol » Tue Jan 08, 2019 11:54 am

Thank you Roger, for the very comprehensive and informative tractation on Noise Figure. It is a pity it is buried in this thread: it would deserve to be uploaded in a separate thread with a more pertinent title, in order to reach and be of use to a larger audience.

glovisol

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