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high IF receiver technique

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neazoi

neazoi

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hello,
I have an IF=500KHz and a 10MHz LO. This means that in my 500KHz filter two signals will appear 10.5MHz and the image 9.5MHz.

1. Can double conversion prevent the image signal?

2. What if I use a double conversion with a high first IF? For example IF=40MHz, LO=10MHz will give the first IF=10+40=50MHz and the image 10-40=? (what will be this, negative?)
 

Negative frequencies are real. They just rotate the other way on a polar chart. The only thing you have to be careful of in a frequency conversion that results in a negative frequency is that you change the "sense" of the modulation, and your receiver can get confused demodulating the signal.

If you double convert RF to -IF1, and the -IF1 to +IF2, they "negative" part cancels out.
 
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    neazoi

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biff44 said:
If you double convert RF to -IF1, and the -IF1 to +IF2, they "negative" part cancels out.
Click to expand...

Could you be more specific on this with an example please?
Thank you very much
 

use image rejection techniques or a SAW filter to remove the image component
 
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    neazoi

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there are some image suppresion mixer with two IF ports, and using a hybird coupler, you can reject image freq. Hittite has many such mixers
 
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tony_lth said:
there are some image suppresion mixer with two IF ports, and using a hybird coupler, you can reject image freq. Hittite has many such mixers
Click to expand...

The image rejection mixer is useable as a first mixer or in direct conversion receivers I think. in a superhet there is one IF frequency so is there of any good to have the image rejection mixer before the IF amplifier?

In the superhet, the wanted signal and the image may present on the IF port, in a sense that sometimes they may overlap even if using a narrow filter.
I have seen somewhere the first IF to be very high (not 455KHz) in order to solve the problem but I do not know how this is actually done.
 

Basically we place a Hartley or a Weaver Image reject mixer after the LNA. Previous techniques involved image reject filters before an Unbalanced diode or FET mixers. If a FET or MOS process is used IF amp is not required as present mixer tech provides a conv gain of 12-20 dB when used in a superhet. Its still called superhet since the signal is boosted inherently by the mixer itself.
 
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    neazoi

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Here is a nice graphical representation of image frequencies, and the reason that negative frequencies are used to make the equations easier to visualize.

RF Cafe - Mixer Image Frequency

Using a higher IF will make the image frequencies to be far away from RF, which will be much easier to filter them.
 
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    neazoi

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This concept is fine but wat if my antenna receives a signal close to RF and having greater signal strength. The Leesons skirt will hide the desired IF with the image IF. This becomes a prob hence image reject filters were used.
 
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    neazoi

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Yes exactly. But a two tone RF can be used for reducing the DC comp at the output.
 

varuniyengar said:
Yes exactly. But a two tone RF can be used for reducing the DC comp at the output.
Click to expand...

That was always confusing to me, thank you.
Since the rejection of the image with an image rejection mixer is very difficult without dsp when talking about large bandwidths (since accurate phasing is difficult to maintain in large bandwidths), it seems to me that this technique is not feasible in direct conversion receivers. Even the R2 receiver from the 90's which uses an image reject mixer needs to be re-set when changing band.
 

As was stated before, negative frequency is simply an anti-clockwise rotation. Because it is negative frequency, we can’t say that it has no physical effect. The physical effect is quite identical to the positive wave, because they are folded in the positive domain.
This is the reason that when using a higher IF frequency than RF, always is used high LO injection (above IF), which leads to a higher image frequency, and better image rejection.
 
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vfone said:
This is the reason that when using a higher IF frequency than RF, always is used high LO injection (above IF), which leads to a higher image frequency, and better image rejection.
Click to expand...

So It should be like the photo, using the low IF in such a configuration for further filtering?
 

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    rf.JPG
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The picture is not very clear for me. What is IF1 and IF2? Where is IF (Intermediate Frequency)?
 
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vfone said:
The picture is not very clear for me. What is IF1 and IF2? Where is IF (Intermediate Frequency)?
Click to expand...

IF1 is the first IF produced by LO-RF. IF2 is the second IF produced by LO+RF. Is that what you mean, maybe I am wrong?
 

neazoi
Ur pic depiction is right but the freqs are adjusted such that IF is << RF and LO
 
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    neazoi

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If I use a higher IF than the RF maximum input frequency then I could remove the image by modest filtering. Then a second mixer could downconvert the filtered high-IF to a lower IF eg 455KHz, where it could be further processed for applying narrower filtering etc.
I am thinkking of usinng saw filters (out from old TV sets) to do the first IF image filtering but I do not know what is their typical bandwidths.
Also is their group delays better than crystal filters, thus better to use in voice communications?
 

The way to do it for a HF Rx is to have a low pass filter at the aerial input with a Fc of 35MHz, and a first IF of > 35, say 40 MHz, and run the oscillator high. So for RF = 1MHz, LO = 41, image = 82MHz. . Then down convert the 40 MHz IF to something like 1.4 Mhz, running the LO high at 41.4, image needs filtering at 42.8. If you use 465KHz, then LO = 40.465 MHz, image at 40.93 MHz, pretty close to wanted at 40. The other BIG problem is adjacent channel interference due to cross mod in the first IF, 40 MHz filters are very wide. If they were not there would be no need for a second IF!
Frank
 

The bandwidth of one channel of TV is always several MHz, but the voice channel are in several KHz, or 10KHz, or 100KHz.
 

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