Help Required in RF Front End

[Ow@i$]

I am designing a HF receiver (10 MHz ) , The front end of the receiver is patched, now since i don't have the facility of a function generator to test whether the front end and tuning portion is responding well on 10 MHz. I want to ask that can a 10 MHz "Square wave" generated from a FPGA kit be used for testing purpose?? as since frequency is 10 MHz, but it is Square Wave NOT Sine Wave. Will all inductor & capacitors, amplifiers will perform correctly on Square wave and not on sine wave, Because in real world Carrier signal is Sine wave!!!

Any help will be appreciated!!!

 

[tony_lth]

You can use a simple low pass filter to change the square wave into sine wave.

 

[Ow@i$]

I know that can be done ... but my question was " Can applying Square wave instead of Sine wave will create issues that I am not supposed to encounter"????

 

[jiripolivka]

I know that can be done ... but my question was " Can applying Square wave instead of Sine wave will create issues that I am not supposed to encounter"????

Using a square or sine wave is irrelevant in receiver testing. Square wave is only different in that it contains harmonics, multiples of 10 MHz.
To test a receiver reliably, the test-signal generator should be insulated from the tested receiver and well screened, so the only way the test signal gets into the receiver is by its input. It is also important to use low-level signal, comparable with the real signal to be received. So a good calibrated attenuator (preferably several steps of attenuation) is preferable.
Many FPGAs, microcontrollers, etc., use 10 MHz or similar frequency as clock, and such signals cause interference to receivers, often quite strong and annoying. Installing the receiver in a screened metal case is important, too, to make sure that receiver output is only generated by signals connected to receiver input (preferably a coaxial connector).

The best way to test a 10 MHz receiver is to use a quartz clock oscillator (sold for less than $5 at e.g. Digi-Key), installed in a small metal case, with a coaxial output connector. The device can be operated with a battery (most clock quartz oscillators are powered from 5 or 3 V) by a push button, so it can have a long life. Screw-on attenuator(s) can be used to reduce a typical output of 3...10 mW to a typical receiver input like 1 microwatt or less.

 

[biff44]

I am going to have to disagree there tex. all those harmonics can fold back into the receiver IF and cause a lot of trouble. Best to filter them out just a little.

and in a worse case, you might blow out a sensitive receiver by giving it too much total input power

 

[jiripolivka]

I accept your disagreement but such receiver which allows harmonics of input frequency affect the output is simply a very bad design. I would rather use good filters in the receiver. What sense is in calibrating a bad receiver?

 

[biff44]

many receivers today, either thru cost cutting or quirks of their zero IF architecture, are "bad" receivers. The trick is to learn how to live with their performance characteristics.

 

[Darktrax]

I am designing a HF receiver (10 MHz )

.. is not nearly enough to know what kind of receiver you mean.

1. Start with is it fixed frequency, or tunable?
2. If fixed, is it the IF (intermediate frequency) for some other front end.
3. What modulation scheme is involved? What kind of signals is it to receive?
4 What sensitivity is expected? What signal levels are involved at the input?
5. Do you have an idea of the dynamic range? This will determine what you intend to use as a signal generator, even if it is just a handy hung-together oscillator.

A signal at exacly 10Mhz, to exceptional accuracy is the WWV time-code signal from NST.
HERE --> 10MHz
It occurs to me a few metres of wire hung out the window gives you a signal any reasonable receiver will hear.

but.. I expect you will need something a little more convenient!
Making up a small crystal oscillator with attenuator to deliver signal levels down to very low levels
is easily searched and found from electronics hobby sites. 10MHz is even within the range of some low-cost
(basically audio) signal generators. Ebay will definitely yield a bit of functional, if old test kit.
You can even connect a crystal across 2 pins of a $5 microcontroller (eg PIC), and apply 5V and 0V to the correct pins, and there will be a strong signal available.

About the square waves. If the levels are low enough that there is no stage driven to distortion, then the basic frequency selection function of a 10MHz receiver will extract only the fundamental (sine) wave.
You help everything in a big way if you give it help - like using Tony's suggestion of a low pass filter.

At HF you will have huge levels of man-made and other noise arriving, if not coupled in from all sorts of electronic kit around you. To use some tuned filter pre-selection to exclude all that noise from frequencies you are not at all interested in is a basic function.
Such a front-end tuned filter will also remove square-wave harmonics.

As you may have appreciated from the other answers posted, making a signal is quite easy, and if it has to be a square wave, you can use it provided you fully understand that it comes with potential problems.

Designing a (good!) receiver is a thing of a entirely different order. It can require the very best of electronic skills, experience and know-how at a challenging level. I am in awe of the guys who can do it well!

 

[Ow@i$]

Thank you all for your guidance !!! So here it what I Understood!!
I can apply a square wave (amplitude in say milli Volts) from a source to the input of my receiver, but preferably i should filter it a bit before applying. ( A op amp Low pass filter will be good i Guess) !!

To Darktrax !!!
Thank for an elaborate answer!!!
1-it is a single frequency receiver.
2-No its not an IF for another front end.
3- you guessed it correct , initially I was thinking of receiving WWV but in My location Pakistan that is, it is not that good!! moreover Chinese being my immediate neighbor also transmits a time signal BPM... so I am thinking of receiving it, I have an old Tunable receiver R-1000, I designed a 10 MHz dipole and received BPM (Not WWV) on it @ 10 MHz !!! now I have to design my own fixed frequency receiver!!
What i found from continuously listening to the signal is that it is not so powerful so the receiver should have good sensitivity!!!
I Am new to Receiver designing I have designed a Receiver for local FM reception but have not designed at this level!!!

 

[Darktrax]

OK - it being a single frequency receiver, life gets easier from here on.

First, you should search (Google?) for "propagation prediction". 10 MHz (30m) can change with time of day, and has specific skip distance from the ionosphere.
Have a look HERE --> LINK

at a North Pole view animation of 10MHz (YouTube). You may find better if you use Use Youtube directly.
If you can place your country on that distorted projection, it may help.

For a receiver , think micro-volts, not millivolts. Get familiar with received power levels
On the video, you see power levels like -133dBW which is -103dBm whic is (I think) 2.5uV in 50 ohms

At 10MHz, the noise floor is so high that the first active device does not have to have a good noise figure. It can be
almost any RF transistor, FET, op-amp. NF=5dB, 8dB, even 10dB will do OK

A handy tool for level conversions is HERE --> **broken link removed**

You do not need anything as awkward as a op-Amp to get the sine wave fundamental carrier from a square wave, in fact, it would be bad!. You would be expecting the first element in the op-amp to be able to handle the huge dynamic range and crashing noise of everything on all wideband frequencies it sees. The whole point about "tuning in" is to exclude as much out of band signal as possible BEFORE the amplifier.

Mainly, you only need a simple coil inductor, in parallel with a capacitor between ground and the antenna input. A lower impedance tapping on the inductor can feed your amplifier. Not much of the square wave will be left. There are of course many variants, but I am sure you can find them.

Still at 10MHz, you could put it through a ceramic or crystal filter to really separate a chunk of the band, or just more tuned circuits.. This filter can be relatively wide, known as a "roofing filter". At this point you have a great many ways to choose. Given that it is fixed frequency, there may not be a need for a superhet down-conversion, but you may do that anyway to then have a signal at some tens or hundreds of kHz. You then can use op-Amp filters. At these frequencies, you can even offer it at a PC sound-card and do some DSP.

Known as "Software-Defined-Radio", or SDR, there is at least one DSP microcontroller where the evaluation circuit party-trick is to connect some antenna, and have FM radio on your PC. You don't have to get elaborate here. Very few parts will yield your receiver.

PS. (Edit)
You don't have to fight hard to get this. A simple search "WWV receiver" yielded many links, low cost kits, eBay and Amazon offers etc. Also, published circuits and advice.
eg. **broken link removed**
and **broken link removed**.

The real experts seem to be the amateur radio ham community at ARRL.

 

[Ow@i$]

Thank you again for your kind help!!!
I will be going for that design and will get back here with the issues!!!
I know About SDR but its a requirement to design it in hardware form !!!!