Receivers by Tony WB6MIE
Alternate title – Receiver system requirements for ham radio narrow band Frequency Modulation reception
Receivers are necessary for Ham Radio communication, so are transmitters, but tonight I am focusing on receivers. There are many types of receiver designs such as regenerative, Homodyne, Super heterodyne and Software Defined Receivers. This discussion will focus on the Super Heterodyne variety as it applies to Narrowband FM reception. I will discuss an older design of single channel receiver, simply because their operation is easier to explain and the concepts of signal reception can be more easily understood.
Since a super heterodyne receiver is a fairly complex device, I will only be discussing the operational aspects of the first stage of the receiver, commonly referred to as the receiver Front End. This tech net was inspired by a question asked by Steve, KJ7RNB. Thank you Steve for the motivation.
In my early days of ham radio, soon after the dinosaurs died and turned into gasoline, getting on ham radio was a little more complex than just getting on the Amazon website and ordering a Baofeng.
Baofeng UV5R ( from users manual- Extended copy)
Receiver Specifications Value
Receiver sensitivity 0.2µV (at 12dB SINAD)
Audio Output 1000mW
Notice the lack of detailed receiver specifications listed above.
You needed to know what band you wanted to operate in and you had to also know what specific frequency inside that band that you were going to use.
Only expensive HF receivers had knobs to tune your frequency of operation.
#3 Tuneable receiver 1960’s
VHF and UHF radios used quartz crystals, commonly referred to as “rocks” because they are basically slices of a quartz rock, to determine their operating frequency. And you got one frequency or channel per crystal. And you needed two crystals, one for your transmitter and one for your receiver. In the case of a multichannel transceiver, you could have as much money invested in your bank of crystals as you had into your radio, sometimes more.
#3 Crystal types used
Crystals are a separate discussion and I will not go any farther into them. They are still an important component of the technology that drives Ham Radio today.
Wikipedia has a great article on crystals but when you google for it, make sure you search for “Crystal Oscillator”, not just “crystal” or you may get distracted by the mystical applications of quartz crystals. That’s all bunk just by the way, but I digress.
Getting back to receivers, a good receiver must have the following attributes:
- Good sensitivity
- Moderately good Dynamic Range, very high dynamic range for repeater applications.
- Good Selectivity, both in its RF and IF systems
- It must be able to convert a received RF waveform to a reasonably accurate copy of the transmitted signal, with minimal added noise, distortion or other artifacts.
A Super Heterodyne receiver uses the following sub systems to accomplish this:
- Radio Frequency (RF) Preselector
- Front end amplifier
- First Local oscillator and mixer
- First Intermediate Frequency (IF) Amplifier and filter
- Second local oscillator and mixer
- Second IF Amplifier and filter
- De-emphasis and roofing filter
- Audio Amplifier
- Speaker/line driver
- A Squelch circuit should also be included.
A squelch circuit is a handy thing to have on a receiver but its importance has diminished recently as most current applications of FM radio require the use of a subaudible tone squelch instead of a noise squelch. A noise squelch is still nice to have in the event that you need to disable your tone squelch so you won’t be blasted with constant noise. You need to be aware of your squelch setting even if you are using tone squelch. It needs to be set fairly loose or wide open for maximum receiver sensitivity. Again, a discussion for another day.
Now I’ve used some big technical words in the previous paragraphs and it’s time for some definitions. I’ll try to take them in their order of appearance as they apply to this discussion.
Lets take Heterodyne first. “A heterodyne is a signal frequency that is created by combining or mixing two other frequencies using a signal processing technique called heterodyning, which was invented by Canadian inventor-engineer Reginald Fessenden.” From Wikipedia. So it’s a technique of mixing two signals, the received RF frequency and a second frequency, generated by a Local Oscillator, this is a signal generated in the receiver itself so we call it a “Local Oscillator”. After you mix the incoming RF signal with the local oscillator in a device called a Mixer we now have a new signal called an Intermediate Frequency or IF. In Super Heterodyne receivers, the heterodyning process is done twice so Reginald decided to call it Super. I would have settled for 2X or XX heterodyne but that’s just me. A super heterodyne receiver can also be referred to as a Double or Dual Conversion receiver. Because that is the process that is going on inside the receiver, frequency conversion to two successively lower intermediate frequencies to make the signal processing easier.
So much for the definitions. And that’s just about as far as I am going go into the signal flow inside a Super Heterodyne receiver tonight. The rest of the downstream stages will be discussed in subsequent tech net presentations in the future.
After a short break, I will begin my discussion of the receiver “Front End”.
Break for questions, radio cool down and to catch my breath.
#4 Front End block diagram
The receiver Front End is the first active amplification stage that the signal sees. It is usually preceded by a pre-selector filter of some type. The pre-selector limits the band of frequencies seen by the front end amplifier to prevent overload of the amplifier by multiple strong signals, such as one would encounter at the base of Black Mountain here in Las Vegas.
#4A Tuneable Motorola Maxtrac Front End
The receiver front end needs to amplify the desired incoming RF signal as much as possible. This sets the receiver’s operating point or minimum detectable signal level.
A typical narrowband FM receiver should be able to open it’s noise squelch on a signal in the vicinity of -123dbm or .16 microvolts and should be well on its way to fully quieted by the time the signal reaches -113dbm or .5 microvolts.
To put this into perspective, the H2 repeater puts between -60 to -70dbm signal level into a hand held receiver over most of the Las Vegas Valley. That’s between 224 and 71 microvolts, not a heck of a lot of power but still way stronger than .5 microvolt.
This requires a fair amount of signal gain while maintaining a good amount of dynamic range.
Dynamic range is defined as an amplifier’s ability to operate over a wide range of input levels, from very weak to very strong while not contributing to signal distortion of any type. It also has to maintain its clean amplification when the signal gets very strong, perhaps even approaching 0dbm. Yes, zero is still a valid quantity in this measurement system, in fact it is our reference point of 1 milliwatt or 224,000 microvolts measured across an impedance of 50 ohms.
In a typical receiver, by the time the desired signal reaches this power level the amplifier device will most likely be into a condition called gain compression or overload. This is when the input signal has driven the transistor output as high as it possibly can go so it can’t faithfully track its input with its output.
Bad things can happen inside the front end amplifier when it reaches this point. It could burn up and stop operating. Excessive noise may be added to the signal, perhaps even blanking out the received signal, a condition not surprisingly called “receiver blanking”.
And an effect called intermodulation distortion will be generated, causing mathematical relationships between other signals in the receiver’s front end pass band to mix in various ways and form product frequencies that may be heard in place of the desired signal. Perhaps some of you have heard strange noises and conversations coming out of your receivers as you drive around town in various places where strong signals exist. This is called intermodulation interference and is an indication of inadequate dynamic range in your receiver’s front end stage.
This is the end of my formal presentation on receiver front ends. In a few weeks I will address local oscillators, mixers and the heterodyne down conversion process.
I would like to catch my breath and let my radio cool down for a little while. I will be open for discussion and questions after a quick trip to the restroom and the kitchen.
Tonight’s audio stream file has been provided by Dave W3QQQ