Tips for Reducing Images

[Roger]

Over the last month there have been numerous posts on this Forum and on Facebook about RSP users having image, "ghost" and spurious problems. In this post I will attempt to outline some of the reasons this occurs and a few tips on how to get better performance out of your RSP.

The RSP is not a direct sampling Software Defined Radio (SDR), like a Perseus or an Icom 7610. Current technology costs limit hobbyist direct sampling receivers to an upper frequency limit of around 50 MHz. The RSP is a DC to 2 GHz. receiver and therefore requires an analog tuner front end, that employs a mixer, in order to shift the band of interest to a lower frequency where it can be sampled, digitized and sent to a digital signal processor (DSP) for demodulation (i.e. AM, FM, USB etc.). It is important to realize that spurious signals like images ("ghosts") and birdies occur to some degree in all analogue receivers that use these non-linear mixers. Prior to SDR it was necessary to use multiple mixers and several intermediate frequency (IF) stages in order to reduce the inherent image and spurious signal problem. If the signal bandwidth is constrained to something like 5 kHz. for AM and 200 kHz. for FM it is possible to get excellent performance. However as the bandwidth of interest increases to 2 MHz. or higher, like in an RSP, the image problem will get much worse and the user needs to understand the limits of the RSP and take steps to alleviate the problem.

So what can be done by the RSP user? The first step is to realize that the RF Gain control (or RF Gain Reduction – default setting) in the RSP2 and RSP1A must be used appropriately. You should only use the minimum level of gain in order to get acceptable reception. Too much gain causes intermodulation products in the RSP and increases the level of the images. I suggest setting it halfway or lower for HF reception and even lower for MW and LF reception. This is particularly important if you live close to a MW or FM transmitter. For VHF you will probably find more gain is required for good signal reception.

The second step is to set the IF AGC to Automatic unless you are prepared to keep a very close eye on it. Experienced users will turn off auto IF AGC and set an appropriate level because this avoids “jumps” in the noise floor due to strong signals appearing on the band (i.e strong ham station in a QSO). The auto IF AGC will adjust the gain of the IF amplifier in order to prevent the A/D converter from being overloaded. You will notice as you increase the RF Gain the IF gain will drop – the two interact. Another reason why the RF Gain control should be used carefully.

Another consideration is the selection of the local oscillator (LO) frequency. In SDRuno and HDSDR it is up to the user to select this frequency. The LO is applied to the RSP mixer on one input and the RF signal (after filtering) is applied on the other input. The output will be a downshifted version of the band of interest but analog mixing is not perfect by any means and unwanted signals will also be output from the mixer as images and birdies. Where these spurious signals occur and the signal level that they attain will depend on many factors including LO frequency, sample rate selected, decimation and the frequency and signal strength of unwanted signal(s) outside the band selected. There is no specific answer to what LO frequency in order to minimize spurious signals in the band of interest. Users will have different environments and what works for one user will not necessarily work for another. In an urban environment there will be MW and FM transmitting stations. There might be an active ham radio operator nearby. Here are some ways that you can select a LO that may work best in your circumstances. You can set the LO above, below or in the middle of a band of interest and see which works best. Lets say we are using a sample rate of 2 MHz. (IF BW = 1.536 MHz.) and listening for shortwave stations on the 31M band from 9.3 to 10 MHz. If we set the LO to 10.1 MHz the RSP would receive from 9.3 to 10.9 MHz. If we set the LO to 9.2 MHz. the RSP would receive from 8.4 to 10. The LO might be set to 9.6 MHz. and reception would be from 8.8 to 10.4 MHz. In all cases you will receive the 31M band but one may have better spurious rejection than the other.

The RSP1A and The RSP2 both have a MW filter which will help to attenuate signals in this band. Unfortunately, this filter does not work when using the Hi-Z port on the RSP2. The attenuation of this filter, like all filters, will have less attenuation at the band edges. So unwanted MW signals can still cause images. So a recommendation for users with MW images is to purchase a low cost MW stop-band filter. These are available for around 10 USD on eBay and Amazon. You can always cascade two of these if you are very close to a MW station.

An alternative to an external MW filter is a Preselector. These have been popular with shortwave listeners for decades. Even expensive analog receivers, used by serious DXers, have utilized one of these to reject out-of-band signals before they enter the receiver thereby improving performance. There are a number of commercial products available or you can build your own.

Another simple box that be built or purchased is a variable Attenuator. You will find this feature built into quality analog communication receivers. It consists of a number of resistors and switches configured to give user selectable attenuation steps. You can set the amount of attenuation for a given situation. This really helps with certain antennas like an active loop or mini-whip because they sometimes have too much gain in their built in amplifier and overload the RSP.

I hope these suggestions help you to get more performance from your RSP. I own several quality HF and VHF communication receivers but I thoroughly enjoy using my RSP and spend more time listening now that I have advanced SDR capabilities in my radio shack.

[RiverPatrol1]

Thanks Roger. Good information and I appreciate anyone trying to enlighten those of us who have just recently acquired a lust for information on the use of SDR's

[KA1OWC]

Thanks! Good stuff...Going to try a couple of your suggestions!

[Radiosammler]

May I copy and translate your post and bring it in the Forum dx-unlimited?

73
Bernd

[Paul]

Thank You Roger
Pure gold for "numpties" like me - very clearly stated.
Thanks for taking the time...

[Roger]

May I copy and translate your post and bring it in the Forum dx-unlimited?

73
Bernd

Yes you can but you must state that the original came from the SDRplay Community Forum.