RadioBanter - heterodyne 455 spurious signal on AM

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heterodyne 455 spurious signal on AM

Hi all,

Radio: Tecsun P230. Problem with AM reception (spurious signal?).
There is a strong AM station 50 kW with frequency 1530 kHz within 10
miles.
Tecsun P230 detects this station when it is tuned to 620 kHz.
No other radio detects this station at 620 kHz.
Actually, other radios detect weak distant stations at this frequency.
Obviously, there is something with heterodyne 1530 - 455*2 = 620

Could you explain what is wrong with radio or just point to the
useful web pages that describe the physical/electrical background
of this phenomena. Probably, I am not interested how to fix the problem,
I just want to understand why it happens.

Thanks a lot in advance,

-Alex

Alexander Kozik wrote:

Hi all,

Radio: Tecsun P230. Problem with AM reception (spurious signal?).
There is a strong AM station 50 kW with frequency 1530 kHz within 10
miles.
Tecsun P230 detects this station when it is tuned to 620 kHz.
No other radio detects this station at 620 kHz.
Actually, other radios detect weak distant stations at this frequency.
Obviously, there is something with heterodyne 1530 - 455*2 = 620

Could you explain what is wrong with radio or just point to the
useful web pages that describe the physical/electrical background
of this phenomena. Probably, I am not interested how to fix the
problem,
I just want to understand why it happens.

Thanks a lot in advance,
--------------------------------
Simplest answer, your radio converts the incoming RF signal
down to a lowr freqeuncy for improved selectivity. In your
radio this is 455KHz.
See: http://en.wikipedia.org/wiki/Image_response
See:http://www.radio-electronics.com/info/receivers/image_response/image.php
The http://www.radio-electronics.co.uk/info/receivers/ is a prettt
good place for beginers to start learning some of the terms
and concepts in radio reception.

Terry

On Fri, 19 Aug 2005 14:39:45 -0700, Alexander Kozik
wrote:

Hi all,

Radio: Tecsun P230. Problem with AM reception (spurious signal?).
There is a strong AM station 50 kW with frequency 1530 kHz within 10
miles.
Tecsun P230 detects this station when it is tuned to 620 kHz.
No other radio detects this station at 620 kHz.
Actually, other radios detect weak distant stations at this frequency.
Obviously, there is something with heterodyne 1530 - 455*2 = 620

Could you explain what is wrong with radio or just point to the
useful web pages that describe the physical/electrical background
of this phenomena. Probably, I am not interested how to fix the problem,
I just want to understand why it happens.

Thanks a lot in advance,

-Alex

Nothing wrong with it. That's how single conversion superhet radios
work.

I used to tune 7415 on my Magnavox AE-3805 by tuning to 6515 (450 kHz
IF).

http://en.wikipedia.org/wiki/Superheterodyne_receiver

Alexander Kozik wrote:

Hi all,

Radio: Tecsun P230. Problem with AM reception (spurious signal?).
There is a strong AM station 50 kW with frequency 1530 kHz within 10
miles.
Tecsun P230 detects this station when it is tuned to 620 kHz.
No other radio detects this station at 620 kHz.
Actually, other radios detect weak distant stations at this frequency.
Obviously, there is something with heterodyne 1530 - 455*2 = 620

Could you explain what is wrong with radio or just point to the
useful web pages that describe the physical/electrical background
of this phenomena. Probably, I am not interested how to fix the problem,
I just want to understand why it happens.

Thanks a lot in advance,

-Alex

Basically it's due to bad design. All radios have an IF that reduces all
incoming signals to a single frequency that then is used for audio
output. If you're hearing an internally produced harmonic, then it
usually means that your radio is suffering from a design flaw, or that
there's some bad parts. Considering how cheaply these Chinese radios are
made, it's not surprising that some would have such poor design and/or
build quality that internal harmonics or "birdies" would be all over the
place. If your radio can't properly handle the IF, then it follows that
SW signals will have images too. Such a radio is practically useless for
DX.

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On Fri, 19 Aug 2005 16:00:47 -0700, running dogg wrote:

Alexander Kozik wrote:

Basically it's due to bad design. All radios have an IF that reduces all
incoming signals to a single frequency that then is used for audio
output. If you're hearing an internally produced harmonic, then it
usually means that your radio is suffering from a design flaw, or that
there's some bad parts. Considering how cheaply these Chinese radios are
made, it's not surprising that some would have such poor design and/or
build quality that internal harmonics or "birdies" would be all over the
place. If your radio can't properly handle the IF, then it follows that
SW signals will have images too. Such a radio is practically useless for
DX.

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http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Not a ''bad design'', just a cheap one.

There are probably close to a billion single-conversion radios with
untuned front-ends out there, providing lots of enjoyment.

running dogg wrote:
Alexander Kozik wrote:

Hi all,

Radio: Tecsun P230. Problem with AM reception (spurious signal?).
There is a strong AM station 50 kW with frequency 1530 kHz within 10
miles.
Tecsun P230 detects this station when it is tuned to 620 kHz.
No other radio detects this station at 620 kHz.
Actually, other radios detect weak distant stations at this frequency.
Obviously, there is something with heterodyne 1530 - 455*2 = 620

Could you explain what is wrong with radio or just point to the
useful web pages that describe the physical/electrical background
of this phenomena. Probably, I am not interested how to fix the problem,
I just want to understand why it happens.

Thanks a lot in advance,

-Alex

Basically it's due to bad design. All radios have an IF that reduces all
incoming signals to a single frequency that then is used for audio
output. If you're hearing an internally produced harmonic, then it
usually means that your radio is suffering from a design flaw, or that
there's some bad parts. Considering how cheaply these Chinese radios are
made, it's not surprising that some would have such poor design and/or
build quality that internal harmonics or "birdies" would be all over the
place. If your radio can't properly handle the IF, then it follows that
SW signals will have images too. Such a radio is practically useless for
DX.

----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

Assuming the radio is a Tecsun PL-230...

It may not be a bad design, it may just be a characteristic of the
design. Alex is describing an image rejection problem on the AM
broadcast band.

In this case it is caused by a very close 50 kW station.

He does not describe what his other radios are, so we don't know what it
is being compared against.

Many AM receivers with a single tuned front end only show 30-40 dB of
image rejection. So, with this portable radio, he may be seeing the
expected results.

Given that the PL-230 is a low end portable, there should be little
expectation for good strong signal performance. I would guess that the
radio is not broken or flawed. It is probably acting as it should.
(Which may or may not be what Alex expected.)

------------

Alex,

An image signal is a signal found at 2x the IF frequency away from its
proper frequency. The radio has a local ocsillator that mixes with the
signals from the antenna. The goal is to change the frequency of the
incoming signal to the frequency of the IF where it is filtered and and
amplified.

The mixing of two signals produces an additional two signals that are at
the sum and difference of the two frequencies. The difference frequency
is what the IF uses.

When tuned to 620 kHz, the oscillator runs at 1075 kHz. The IF is the
difference, or 455 kHz. But signals at 1530 kHz also result in the
difference being 455 kHz. The mixer converts both 620 kHz and 1530 kHz
to the IF frequency.

It is up to the radio to filter out the unwanted frequency before it
gets to the mixer. If it is successful, then you will only hear the
desired 620 kHz signal.

However, you have a very strong signal at 1530 kHz. Even if the radio
eliminates 99% of the signal before it gets to the mixer, the remaining
1% may be strong enough to hear and may prevent you from hearing a
weaker station on 620 kHz.

Images can be a problem on any radio. You will see it as a common spec
that is frequently measured by manufacturers and reviewers. Methods to
reduce the problem include more filtering before the mixer to reduce the
unwanted signals, or the use of a higher IF frequency. The higher IF
helps because the image is further away from the desired signal so it is
easier to filter out. With a high IF, multiple IF frequencies
(additional mixers and IF stages) are used to provide the selectivity
that is harder to get at the high frequencies.

Only you can decide if this is a problem for you. If you see images from
AM stations that are much weaker where you are at, then you may have a
problem with the radio, but if it is just from very strong station, then
you may have to live with it or use a different radio.

Hope this helps,

craigm

We have a similar 50 kW station here in Tucson on 1550 kHz and all of
my single conversion radios with a 455 kHz IF will pick it up at 640
kHz. This is typical for single conversion and a local strong station
on the high end of the band.

Frank

The heterodyne design (expecially the super heterodyne design) was a big
advancement in radios many years ago,wasen't it?
cuhulin