I have a Sangean ATS-803a and a Degen DE-1103. On both, I get images in
the LW band of MW band stations. They are at the frequency of the medium
wave station divided by 10, i.e. 1010 khz shows up at 101. Why do they
show up at 1/10 the frequency? These are both dual conversion radios.

The circuitry that tunes, is oversensitive when the antenna picks up too much signal and overloads the radio. My Dx394 has two different connections on the back. I get the same trouble somtimes and just switch the connection, using the same antenna. I don't know as much as the hams do about the workings of that circuitry, but if you can reduce the antenna length a bit when that happens, it might help. You need to attenuate the signal strength a bit. I also have trouble in 19 meters with local FM stations depending on which connection I use, and the cure is the same - I switch where I have the antenna connected. The dx 394 does have an attenuator switch and I will use it on occasion. I have noticed that if I am listening to a sideband signal that is particularly strong, backing down on the gain control helps in trying to clarify the signal. HTH mm

God gives Peace not war +

"Conan Ford" wrote in message .159...
I have a Sangean ATS-803a and a Degen DE-1103. On both, I get images in
the LW band of MW band stations. They are at the frequency of the medium
wave station divided by 10, i.e. 1010 khz shows up at 101. Why do they
show up at 1/10 the frequency? These are both dual conversion radios.

My 1103 generates images of strong stations at twice the if or 900khz
down. I've found mw images down in the lw band, 6mhz images in the
5mhz band, 9mhz images in the 8mhz band. The radio is a lot of fun to
use but the images can be a little irritating at times. Especially
when the cover up some non-broadcast signal.

Conan Ford schrieb:

I have a Sangean ATS-803a and a Degen DE-1103. On both, I get images in
the LW band of MW band stations. They are at the frequency of the medium
wave station divided by 10, i.e. 1010 khz shows up at 101. Why do they
show up at 1/10 the frequency? These are both dual conversion radios.

That is not (1010/10) kHz = 101 kHz, but (1010 - 2x 455) kHz = 100 kHz.
Non-ideal 2nd IF image rejection isn't uncommon particularly among
smaller and/or less expensive rigs with high 1st IFs (Sony's
ICF-SW7600[|G|GR] models are also affected). The crystal filters used
there should be selective enough by themselves, but apparently leakage
around the filter (better receivers don't have rather large ground
planes around the 1st IF filters for no reason - RF tends to go its own
ways as you get to higher freqs) and possibly also mixer related issues
limit ultimate rejection. Single conversion sets with frontend tracking
(varicap tuned in PLL based sets, with conventional tuning capacitors
otherwise) may actually be in advantage on low frequency ranges like MW
and LW, that's why high performance AMBCB sets have never needed to use
dual conversion. (In fact, one more mixer may also mean more noise.) PLL
based dual conversion designs are usually wideband receivers without a
lot of front-end filtering. (Bandspread dual conversion analogs are a
different matter, these can easily use band filters for SW. That's why
they get along with much lower 1st IFs.) I guess including additional
front-end tracking on LW/MW only would cost even more than getting 1st
IF filtering right. Better-quality portables use switched front-end
filters (the Satellit 700 even had a tracking function, not always that
well aligned BTW), but these require care in application as well, given
switching diodes used for selecting a filter electronically can
introduce intermod themselves! (One of the Kenwood R-2000 mods comprised
the replacement of the stock switching diodes with, I think, PIN types.
Actually a number of older rigs will benefit from such a measure,
including the JRC NRD-515.)

Stephan
--
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This is a SCSI-inside, Legacy-plus, TCPA-free computer

That is correct. Another thing that can cause that (2 X 2nd I.F.) image is
2nd LO leakage coming back into the input of the 1st mixer. Once the concept
of mixer theory is understood, things become clear. I know that many, if not
most of the folks on this group understand this, but for the new folks, I
will repeat it.
A mixer is a three port device. It doesn't matter if it is used as an SSB
detector or as the first stage in a superheterodyne receiver; the concepts
are the same. In the first stage of a receiver, the antenna input is applied
to the RF port of the mixer, the tuning oscillator is applied to the LO
port, and and the resultant signal is taken from the IF output port. Here is
where it gets a little tricky (not really!). If you could see the IF output
of the mixer on a spectrum analyzer, you would see three distinct signals.
You would see a lower sideband signal, the LO signal (suppressed by a
certain amount, depending on the LO feedthrough), and an upper sideband
signal. Both the lower sideband and the upper sideband will be separated
from the LO feedthrough signal by the intermidiate frequency. This means
that if you have a 455kHz I.F., the lower sideband will be 455kHz BELOW the
LO signal, while the upper sideband will be 455kHz ABOVE the LO signal. If
you have now preselection ahead of the mixer, the receiver will respond
equally well to both of those sidebands. This is why if you are tuned to
580kHz, for example, you can also receive a signal on 1490kHz. This is not a
defect of the receiver; instead, it is a characteristic of this type of
receiving system.
On lower frequency receivers, high-side LO injection is used. This means
that the lower sideband is the desired sideband. In this case, the upper
sideband is the image.
Now, in the case of the DX394, the 1st I.F. is 45MHz. The 1st LO operates
from 45 to 75MHz. This means that the receiver can conceivably respond to
two bands, depending which sideband (or band of frequencies) is selected by
the preselector that is ahead of the 1st mixer. The desired band of
frequencies is 0 to 30 MHz, but the upper sideband range of frequencies is
90 to 120MHz. This explains why this receiver can hear some FM band
responses in the 19 Meter band (15MHz LSB.................105MHz upper
sideband). This can be cured by additional low-pass filtering ahead of the
front end, of better shielding of the receiver from external signals.
In the case of an SSB detector, the 455kHz (or 450kHz in some cases) I.F.
signal is applied to the RF port of a mixer, the BFO signal is applied to
the LO port, while the audio is recovered from the IF port. You still have
an upper sideband and a lower sideband signal, except these signals are now
at an audio frequency. There are three ways to select the desired signal in
this case (there may be others).
The first way is to use a 455kHz BFO signal, and use ceramic, crystal, LC,
or mechanical filters ahead of the SSB detector. With a 2.3kHz bandwidth,
the LSB filter will typically be centered at 453.5kHz, while the USB filter
will be centered at 456.4kHZ.
The second way (and the more common way) is to use a filter centered at
455kHz, and use two different BFO frequencies of 453.6kHz and (you guessed
it) 456.4kHz. This is done either by using two different resonators in the
BFO circuit or by using one resonator and warping it with either a varactor
diode or a trimmer capacitor.
The third way (less common) involves using the phasing method to eliminate
the undesired sideband.
Finally, concerning that 1st mixer..................as far as I know,
there are only two ways to eliminate that primary image. The first one
involves using either a lowpass (when an upconversion scheme is used), a
bandpass, or a combination of the two.
The bandpass can either be a tracking type or a fix tuned type.
The second way involves using a Quadrature image-reject mixer, the way that
Drake does it with the R8 receiver. This technology has been around for a
long time, being used by Hallicrafters in their SSB transmitters (the
phasing method of sideband generation).
I hope all of this LONG explaination helps to clarify at least some of the
questions you might have.
Remember, just because a receiver is dual conversion..........it doesn't
mean that it will totally reject that 910kHz image. If you look at the
crystal filter specs, a 910kHz rejection factor is usually given. For a
single monolithic two-pole filter at 45MHz, this factor is usually
about -50dB, while a 4-pole set is usually about 60dB, and an 8-pole filter
is around 80dB. This "roofing" filter is what usually determines the
close-in IP3 rating, since the 2nd mixer characteristics are the dominating
factor here.
If this isn't enought, feel free to e-mail me directly, and I can explain
this stuff in more detail!
You're all a great bunch a folks!

Pete

"Pete KE9OA" wrote in message
...
That is correct. Another thing that can cause that (2 X 2nd I.F.) image is
2nd LO leakage coming back into the input of the 1st mixer. Once the
concept of mixer theory is understood, things become clear. I know that
many, if not most of the folks on this group understand this, but for the
new folks, I will repeat it.
A mixer is a three port device. It doesn't matter if it is used as an SSB
detector or as the first stage in a superheterodyne receiver; the concepts
are the same. In the first stage of a receiver, the antenna input is
applied to the RF port of the mixer, the tuning oscillator is applied to
the LO port, and and the resultant signal is taken from the IF output
port. Here is where it gets a little tricky (not really!). If you could
see the IF output of the mixer on a spectrum analyzer, you would see three
distinct signals. You would see a lower sideband signal, the LO signal
(suppressed by a certain amount, depending on the LO feedthrough), and an
upper sideband signal. Both the lower sideband and the upper sideband will
be separated from the LO feedthrough signal by the intermidiate frequency.
This means that if you have a 455kHz I.F., the lower sideband will be
455kHz BELOW the LO signal, while the upper sideband will be 455kHz ABOVE
the LO signal. If you have now preselection ahead of the mixer,

Sorry folks, that should be "if you have NO preselection ahead of the mixer"

Pete

Thanks for the explanation

God gives Peace not war +

"Pete KE9OA" wrote in message
...

"Pete KE9OA" wrote in message
...
That is correct. Another thing that can cause that (2 X 2nd I.F.) image is
2nd LO leakage coming back into the input of the 1st mixer. Once the
concept of mixer theory is understood, things become clear. I know that
many, if not most of the folks on this group understand this, but for the
new folks, I will repeat it.
A mixer is a three port device. It doesn't matter if it is used as an SSB
detector or as the first stage in a superheterodyne receiver; the concepts
are the same. In the first stage of a receiver, the antenna input is
applied to the RF port of the mixer, the tuning oscillator is applied to
the LO port, and and the resultant signal is taken from the IF output
port. Here is where it gets a little tricky (not really!). If you could
see the IF output of the mixer on a spectrum analyzer, you would see three
distinct signals. You would see a lower sideband signal, the LO signal
(suppressed by a certain amount, depending on the LO feedthrough), and an
upper sideband signal. Both the lower sideband and the upper sideband will
be separated from the LO feedthrough signal by the intermidiate frequency.
This means that if you have a 455kHz I.F., the lower sideband will be
455kHz BELOW the LO signal, while the upper sideband will be 455kHz ABOVE
the LO signal. If you have now preselection ahead of the mixer,

Sorry folks, that should be "if you have NO preselection ahead of the mixer"

Pete

Anytime!

Pete

"ShortwaveMan" wrote in message
ink.net...
Thanks for the explanation

God gives Peace not war +

"Pete KE9OA" wrote in message
...

"Pete KE9OA" wrote in message
...
That is correct. Another thing that can cause that (2 X 2nd I.F.) image
is 2nd LO leakage coming back into the input of the 1st mixer. Once the
concept of mixer theory is understood, things become clear. I know that
many, if not most of the folks on this group understand this, but for the
new folks, I will repeat it.
A mixer is a three port device. It doesn't matter if it is used as an SSB
detector or as the first stage in a superheterodyne receiver; the
concepts are the same. In the first stage of a receiver, the antenna
input is applied to the RF port of the mixer, the tuning oscillator is
applied to the LO port, and and the resultant signal is taken from the IF
output port. Here is where it gets a little tricky (not really!). If you
could see the IF output of the mixer on a spectrum analyzer, you would
see three distinct signals. You would see a lower sideband signal, the LO
signal (suppressed by a certain amount, depending on the LO feedthrough),
and an upper sideband signal. Both the lower sideband and the upper
sideband will be separated from the LO feedthrough signal by the
intermidiate frequency. This means that if you have a 455kHz I.F., the
lower sideband will be 455kHz BELOW the LO signal, while the upper
sideband will be 455kHz ABOVE the LO signal. If you have now preselection
ahead of the mixer,

Sorry folks, that should be "if you have NO preselection ahead of the
mixer"

Pete

Stephan Grossklass wrote in
:

Conan Ford schrieb:

I have a Sangean ATS-803a and a Degen DE-1103. On both, I get images
in the LW band of MW band stations. They are at the frequency of the
medium wave station divided by 10, i.e. 1010 khz shows up at 101.
Why do they show up at 1/10 the frequency? These are both dual
conversion radios.

That is not (1010/10) kHz = 101 kHz, but (1010 - 2x 455) kHz = 100
kHz. Non-ideal 2nd IF image rejection isn't uncommon particularly
among smaller and/or less expensive rigs with high 1st IFs (Sony's
ICF-SW7600[|G|GR] models are also affected). The crystal filters used
there should be selective enough by themselves, but apparently leakage
around the filter (better receivers don't have rather large ground
planes around the 1st IF filters for no reason - RF tends to go its
own ways as you get to higher freqs) and possibly also mixer related
issues limit ultimate rejection. Single conversion sets with frontend
tracking (varicap tuned in PLL based sets, with conventional tuning
capacitors otherwise) may actually be in advantage on low frequency
ranges like MW and LW, that's why high performance AMBCB sets have
never needed to use dual conversion. (In fact, one more mixer may also
mean more noise.) PLL based dual conversion designs are usually
wideband receivers without a lot of front-end filtering. (Bandspread
dual conversion analogs are a different matter, these can easily use
band filters for SW. That's why they get along with much lower 1st
IFs.) I guess including additional front-end tracking on LW/MW only
would cost even more than getting 1st IF filtering right.
Better-quality portables use switched front-end filters (the Satellit
700 even had a tracking function, not always that well aligned BTW),
but these require care in application as well, given switching diodes
used for selecting a filter electronically can introduce intermod
themselves! (One of the Kenwood R-2000 mods comprised the replacement
of the stock switching diodes with, I think, PIN types. Actually a
number of older rigs will benefit from such a measure, including the
JRC NRD-515.)

Stephan

Interesting, I'd seen a lot of references to LW station really being MW
stations on this group, but not a detailed explanation like this. I had
always thought that dual conversion took care of this. I did always
wonder on my (even lower end) dual conversion DE-1102 why the MW
broadcast station bled into SW bands.

For the DE-1103, I know there is some filtering switching in for SW
frequencies, in particular a lowpass filter for 1700 khz or so, and a
highpass filter for 30000 khz. This gets rid of the MW band bleeding
into the SW bands, which you see on other radios like the DE-1102 (dual
conversion) and the PL-550 (single conversion). I suppose the ATS-803A
must have some similar scheme as well. I imagine that there wasn't as
much care or concern about the LW band when these radios were designed.

"Stephan Grossklass" wrote in message
...
Pete KE9OA schrieb:

Another thing that can cause that (2 X 2nd I.F.) image is
2nd LO leakage coming back into the input of the 1st mixer.

Happen to have a little example with numbers? I still have difficulty
imagining this.

(see rant below)

Let's take a typical rx with 55.845 MHz 1st IF and 455 kHz 2nd IF, plus
a desired frequency of 10.000 MHz. Furthermore, 1st IF filtering be
perfect in suppressing 910 kHz images and a SW lowpass be present.
1st LO = 65.845 MHz.
2nd LO = 55.390 MHz = const.

Even if there is some 2nd LO leakage into the 1st mixer input, it should
be treated exactly like any other ordinary signal (with a signal level
much smaller than that of the 1st LO), i.e. mixed to an unexciting
10.455 MHz. Obviously for another signal to be observed as a +910 kHz
image we need to have it on 54935 kHz, after the roofing filter, but I
assumed this would be perfect. Alternatively, we'd need to generate a
10910 kHz signal in the 1st mixer and leak that back to the 1st mixer's
input (but how likely is that?). I guess I'll grab a piece of paper
now...

Stephan

You would think that it would be the case, but it just isn't..............my
first dual conversion receiver had a 1st I.F. of 10.7MHz and a 2nd I.F. of
455kHz. Low-side injection of the 10.245MHz 2nd LO at a level of +7dBm into
a diode ring mixer.
Upper sideband = 10.7MHz................lower sideband = 9.79MHz. I had a
910kHz rejection of only 35dB, which is pretty bad for this type of design.
I went through all of the calculations and had the folks at Piezo Technology
stuck. They never heard of such a thing. I tried different filter
termination impedance, tuned circits at 10.7MHz, atteunator pads to provide
a resistive termination for the crystal filter. It was out of desperation
one day that I tried a copper shield around the 2nd LO and the image dropped
down below the noise floor of the system. The offending culprit was WOPA
1490 interfering with WILL 580. WOPA was in Oak Park, Illinois, while WILL
is in Champaign, Illinois, at the University of Illinois.
Once I went to lower power 2nd LOs the problem hasn't shown up. Also, with
the selective diplexer I am using after the 1st mixer (BW =13kHz @ 6dB), the
additional selectivity helps out quite a bit.
When I told some of my co-workers at Collins Radio Co. about it, they hadn't
heard of this mechanism either.

Pete