Superheterodyne LO stability,power ,Mixer input impedance?

[causeitso]

Hi , I have exactly three questions about superheterodyne receivers .

They may seem at first like simple ordinary questions , but I searched left and right and didn't find any satisfactory answer.

Question (1)- In a superheterodyne receiver I know that to convert the RF to IF we mix the RF with an LO of a predetermined frequency , that's ok.
But during my simple idealistic simulation of an RF modulated signal of say (110 MHz) multiplied by an LO of (100 MHz) the result (information) is at (10 MHz).
The simulation ran so smoothly and I could recover the information exactly as it was transmitted (whether it is ASK , FSK or PSK).
But to test the non-ideal behavior of the receiver I changed the LO frequency very slightly (100.001 MHz) , this very slight change caused a disastrous effect on the demodulated (information) signal , which manifests it self as if the information signal is now riding (or being multiplied by) a very low frequency , and this low frequency periodically Nulls the information signal.
The more accurate the LO frequency say(100.0001 MHz) the lower the frequency of this interfering signal .
The problem is that getting so incredibly unrealistically close to a LO frequency of (100 MHz) doesn't even begin to solve the problem , as this creates a so incredibly low frequency signal that will Null the information sooner or later.
This Nulling happens when the RF and LO are out of phase by 90 degrees.
Do you see what I am talking about?
So how could we avoid this problem (knowing that I didn't see any tutorial on superheterodyne receiver address this problem as if it doesn't exist).
Plz , Correct my understanding if I am wrong.


Question (2)- In mixer design they talk about LO input power and IF input power which affect the 1-dB compression and TOI.
How could the input LO power be calculated , I mean the LO has an output resistance and the mixer have a nonlinear input resistance , so is the LO input power is the power dissipated on the output resistance of the LO?
Or is it the power that get dissipated on the input resistance of the mixer or what?
Same goes for IF input power.


Last Question- If I am set to design a Double-balanced mixer (4 diodes) , then what BALUN to use (1:1 , 1:4 etc..).
I know that the choice depends upon the required impedance matching , but the problem is that the diodes have nonlinear impedance , so how could I know the input impedance of the diodes?
Is this DBM balun design has a rule of thumb or something , because I figured that different diodes are not really that different .
So is there a general Balun ratio used for DBM that use silicon diodes (0.7 volt forward bias voltage).

Sorry for these long questions , but I wouldn't ask if I had of any other possible option.

Thank you.

 

[LvW]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

The more accurate the LO frequency say(100.0001 MHz) the lower the frequency of this interfering signal .

Question: Is this effect - resp. the frequency of the interfering signal - dependent on the chosen baseband frequency ?

 

[echo47]

Is your receiver continuous-time (analog) or discrete time-sampled (digital)?

When you multiply 110 MHz by 100 MHz, you get both 10 MHz and 210 MHz. Be sure to filter out the 210 MHz. If your system (or simulation) is time-sampled, the 210 MHz product could alias down and cause great confusion with your 10 MHz IF. The confusion usually appears as an ugly beat frequency. If your mixing product frequencies, phases, and sample rate are just right, the ugly beat frequency becomes zero hertz and may seem to disappear. That scenario seems to match your symptoms.

I don't know your other two questions.

 

[causeitso]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

thank you.

LvM , No the frequency of the interfering signal is dependent upon the difference between local oscillator frequency and the RF carrier frequency.
eg (100.001 MHz-100 MHz=.001 MHz).

My receiver is an analog receiver , i attached a picture of the simulation.
You notice when the LO frequency is (100.1 MHz) the demodulated signal experiences nulls every now and then.

The real problem is that even if the LO frequency got arbitrarily close to (100 MHz) the nulls would still occur but with a greater distance from null to null.

So how could practical real receivers avoid this fundamental problem?

 

[LvW]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

As I can see from your picture the input carrier frequency is 100 MHz (AM modulated with 1 MHz). Is this correct ? And the LO also runs at 100 MHz ?

 

[causeitso]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

Yes this is correct,
AM Carrier frequency=100 MHz
AM modulating frequency=1 MHz
Modulation index=1
Local oscillator frequency=100 MHz

 

[echo47]

By changing the IF to zero hertz, you now have a direct-conversion receiver. That requires a quadrature LO and quadrature mixer to preserve the modulation information which is now centered around zero hertz. Also, you need to properly interpret the mixer's complex output.

I did a quick Google search and found lots of examples.
Here's one. Look for "Zero IF (Direct Conversion)" towards the bottom:
**broken link removed**

If you are only doing simple AM detection, there may be a shortcut technique that I haven't seen.

 

[causeitso]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

Yes , this is a direct conversion receiver , but the problem I am talking about here would still occur with double or triple conversion receivers.

So my question is: can I get away with an LO frequency that doesn't exactly (theoretically) match the Rf frequency (for direct conversion) or match the IF frequency (for double conversion)?

 

[echo47]

If your IF bandwidth includes zero hertz, then you need a quadrature LO and mixer at that stage. It doesn't matter how many other IF stages the receiver has.

Yes you can have slightly unequal LO and RF. Quadrature techniques preserve the IF spectrum even when it's around zero hertz. A simple LO and mixer would be inappropriate because it mirrors the IF spectrum around zero hertz, causing the undesirable interference effects (nulls) you see in the simulation.

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

1. You have simple beating between carrier frequency and unequal LO signal. You need to use IF filter to extract your required frequency. Mixers are not good for AM demodulation, they are better for DSB and SSB.

2. It is rare case in RF, but you can use input voltage from LO across mixer input resistance to determine mixer's input power. Usually power is determined through power budget calculations. You need consider matching also. If you want to determine mixer input impedance it will depends on mixer circuit. Usually there are two points: when mixing element is on and when it is off. You may use the geometric means from these two numbers. For balanced mixers this can be done for each pair for corresponding half of period.

3. See step 2.

 

[RFDave]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

Lot's of dancing around this answer. In short, as the LO frequency is offset from the ideal frequency, your BER will degrade. Cellular standards have transmit frequency accuracy standards that are less then 1 PPM (1 Hz error per 1 MHz frequency) to address this.

Dave
www.keystoneradio.com

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

To RFDave,

Talking about the first question it is very simple case. AM have nothing to do with BER and its degradation. LO instability consideration is also not for this case. Anybody who try to use SSB receiver to listen AM station knows this effect.

 

[causeitso]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

Thank you very much for your helpful answers.
Although i didn't fully understand the problem but i think i am on my way to something.
thank you all especially RF-OM.

 

[echo47]

It may help you to visualize the signal's spectrum during mixing. A 100 MHz signal with 1 MHz AM has components at 99 MHz, 100 MHz and 101 MHz. If you mix that down to zero hertz, the components move to -1 MHz, 0 MHz, and +1 MHz. When using an ordinary (non-quadrature) mixer, the -1 MHz and +1 MHz components both become 1 MHz and interfere with each other, causing the ugly beat frequency you saw when you slightly detuned the system. If you use a quadrature mixer, the -1 MHz and +1 MHz components remain distinct, and don't interfere with each other.

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

But why it is necessary to use quadrature or image reject mixers with AM? Why you guys always try to use digital modulation concepts to this simple analog case? AM is AM and that is it. Your advises may confuse person who ask help. Let him to understand what is going on with his experiments and then he will be able to go ahead. If he ask about digital modulation then you may explain it.

 

[echo47]

I suggested it because I think it's a good way of understanding why the malfunction occurred in the given zero-IF receiver, from a spectrum point of view. Zero-IF AM detection can work fine. I agree that other AM receiver architectures may be simpler or more economical.

I don't see how "digital modulation" pertains to this discussion.

 

[biff44]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

You need to add a lowpass filter, either analog or digital, at the mixer output. You are probably having aliasing problems with the 210 MHz and any non-linear intermod outputs.

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

Just because quadrature mixers are used for digital modulation, not for AM.

 

[echo47]

A quadrature mixer simply shifts the frequeny band -- the type of modulation doesn't matter. I've used them successfully in AM systems. Those systems already had quadrature mixers for other purposes, so there was no cost impact.

 

[RF-OM]

Re: Superheterodyne LO stability,power ,Mixer input impedanc

Quadrature mixer always produces two output signals instead of one. It may lead to much undesired consequences in your system. It is not well done design if you deliberately do such things. These mixers were designed for digital modulation and should be used only with digital modulation. Using them for AM is absolutely unacceptable and poor design. Your goal is to suppress all unwanted signals into system but not to created additional.