Intermod and images on my UBCD536-PT?

[RidingDownUnder]

Hi gang,
a few months ago I purchased a new UBCD536-PT (Australian model) scanner and am amazed by the nil existence of intermodulation and images from high power local transmission. I have never had to use the IFX mode on my radio.

I only got back into scanning and I can remember my previous Ham radio’s all brands, Unidens, AOR and RS scanners from the 80-2000’s were infected with internally generated noises and out of band images from high power transmissions.

I know as time advances so does technology and I think Uniden has done a great job in their R&D department to bring us a satisfactory product.

This is my first Digital Trunking DMA scanner and I am totally impressed by its performance. P25, NXDN and DMR… In Sydney we have no simulcast so I won’t comment on this.

A question, has anyone with a 536HP ever had problems with intermod or images?

Thanks

Anthony.

 

[pb_lonny]

My experience (I don't own one but did have a lend of one for a couple of weeks) was that it was excellent. My UBCD325P2 is also great.

 

[Ubbe]

Intermod and image problems from high power signals are usually dependent of the sum of all signals received. BCD536 have 7 different band filters that then can be made to cover a more narrow frequency range in each filter. The more filters you have the better the reception.

BCT15/BCD536/996 are excellent receivers that can handle a problematic RF situation. Uniden had UBC780/785 that used electronically tuned filters by using varactor diodes that tuned the filter to the frequency received that also worked well but was a too expensive solution and where abandon for future scanners. Uniden probably discovered that maybe 95% of users do not need a receiver with that kind of high performance, that more or less equals a professional 2-way radio, but are also more expensive to manufacture.

/Ubbe

 

[RidingDownUnder]

OK I spoke too soon. I put on a 800-900MHz antenna with a bit of gain and found a image. Originally it’s a Community Radio service that is very close to me as I can hear it without a antenna. Transmitting frequency is 151.800 with full bars and imaged on 912.950MHz with no signal strength. If there’s 1 I found then there will be more. Since using this new 800-900MHz antenna I found a monitor on 916.000MHz with CTCSS 67.0Hz. it’s transmitting 24/7. I can hear a baby so it’s a monitor. When I hear a car go past my house I then hear it on this frequency about 500m/s later, so it’s close. I didn’t know baby monitors were on and around 916MHz.

 

[Ubbe]

Transmitting frequency is 151.800 with full bars and imaged on 912.950MHz

Set IFX to the 912.950 frequency and see if it the signal disappears. Open the squelch and listen. If it's still there then it's probably a signal in the air. Could be a link that are used for that 151.800 transmitter or a mix with another transmitter creating the 912 signal. You antenna only have gain in the 800-900MHz range and no improvement in the 150Mhz range that would increase the 150Mhz signal?

I could check my 536 using a strong signal from a signal generator, but then your 536 could be using new hardware that behave differently and that intermod frequency could appear somewhere else in my scanner.

The filter in the scanner for the 750-1300MHz band and not very effective but still attenuates a 150MHz signal 500 times, 25dB.

/Ubbe

 

[RidingDownUnder]

Hi Ubbe,
thanks for your information. This exactly same thing happened on my Kenwood TH-79D dual band portable.

Where our ‘local’ ethnic Community Radio Stations are transmitting on the 151-153MHz NFM bands, Iget images on 800MHz.

Now after more listening I have found about 6 images in the 800MHz ALL coming from the local 151-153MHz Community Radio local transmissions. There’s one particular station that has 2 images in the 800MHz.

The images are weak and don’t interfere with any of my listening.

Thanks
Anthony.

 

[RidingDownUnder]

Ubbe,
I set the IFX to 912.950 and it does stop the image on that frequency. It’s the first time I’ve used this feature and am surprised on it’s effectiveness. I tried IFX on 3 other close images near 912 MHz and it stop them aswell. I’ve done a complete scan of all frequencies and didn’t find any other images. So the BPF in the 750-1300 is very poor as you mentioned.

A question, do you know the IF’s spec’s of the 536 and is it triple conversio..

 

[Ubbe]

It's probably the same as BCT15x and SDS200 that both use one IF filter for 380.75MHz and the other at 265.55MHz for the first IF.
2:nd IF for BCT15x are at 10.8MHz and then 450KHz.

It's probably the mixer that is the problem as the first amplifier seems to be able to handle high signals like -20dBm but the mixer starts to compress input signals at -50dBm and OP3 at -40dBm. That mixer IC are obsolete so in recent manufactured scanners it's using another IC that I do not know if it worse but hopefully it's better. If that 750-1300MHz filter attenuate 25dB at 150MHz then you'll need a huge signal like -25dBm to create problem in the 750-1300MHz range, theoretically.

But then you also have the sum of all signals and that's why it sometimes help to use a FM trap filter to at least reduce the signal from a high power broadcast transmitter to get below the critical level where problem starts. Also if a cellular or television filter can be used it could also help.

In comparison a SDS100 have its problematic level starting at -60dBm and gets real bad at -40dBm, something like 35dB worse compared to other Uniden scanners.

If you look at antenna amplifiers they mostly have a compression of signals at an input of 0dBm and IP3 of an input of +20dBm. So intermod and similar issues are mostly a receiver problem.

/Ubbe

 

[pb_lonny]

Glad it was able to fix the issue for you

 

[RidingDownUnder]

Yes thanks pb.

Thanks Ubbe for all your input, you are very knowledgeable when it comes to RF communications and it is a very interesting field.

You said,

”It's probably the same as BCT15x and SDS200 that both use one IF filter for 380.75MHz and the other at 265.55MHz for the first IF.”

I thought the SDS series are 100%SDR, but I did read on here somewhere that they are a hybrid ?

Thanks for your help

Anthony

 

[Ubbe]

I thought the SDS series are 100%SDR, but I did read on here somewhere that they are a hybrid ?

The manufacturer Rafael says: "The R836 is a highly integrated silicon tuner that builds in low noise amplifier (LNA), mixer, fractional PLL, VGA, voltage regulator and tracking filter"

It's a tuner and downconverter and it outputs an IF signal of some 2,5MHz and there's no digital sampling of the signal in the receiver chip. The digital part happens in the sub processor DSP that digitize the analog 2,5MHz signal and works as the demodulator.

/Ubbe

 

[RidingDownUnder]

Very interesting. That’s the same IC they use in a RTL SDR Dongle. I wish Uniden would give schematic diagrams like Yaesu with their products because I find them interesting and good for fault finding. Maybe they don’t because too many 27MHz’s owners like to tinker with their radios. But I suppose not having a schematic hasn’t stopped them.
Thanks.

 

[Ubbe]

RTL-SDR dongles, and probably all dongles that used the R820T receiver chip that are now obsolete, use a R860 receiver that they say are identical.

Even the R836 where only used by Uniden in the first years of SDS100 and when SDS200 where released it was obsolete and from then on both SDS100 and SDS200 use a R840 receiver chip that are identical to R836 but has more features for media use, like 8k picture resolution and more audio surround modes.

/Ubbe