What makes scanners so big?

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Mike_G_D

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To help answer the OP's question, at least partly, answer the following very simple math questions:

1) What is 300 divided by 25?

2) What is 300 divided by 824?

3) What is the larger of the two?

The answer to the first question is the longest wavelength in meters that a scanner radio must deal with and the answer to the second question is the longest wavelength that a cellular phone must deal with (disregarding internal conversion stages, etc.).

Unlike digital electronics radio frequency (RF) electronics are often subject to physical size issues due to the wavelenghts of the radio frequencies being dealt with. In some cases the lower frequencies require larger components. If you want to design a good robust radio receiver that can deal with a wide spread of frequencies between 25MHz and 2GHz you need to allocate some space to handle those lower frequencies well. Yes, there are really tiny receivers that do have huge frequency ranges going down into the 100's of KHz BUT they really won't work great when presented with a heavy polluted RF environment with a lot of strong local signals presented to their antenna ports. In other words, you can reduce the size of the radio and still handle lower frequencies but at some compromise in performance.

Here's another consideration - these radios use superheterodyne architectures inside. What that means is there are internal stages which convert the radio frequencies being received eventually down to a lower frequency so that the rest of the "stuff" only has to handle that lower frequency; they do this by mixing the incoming radio energy with locally generated radio energy. That locally generated energy is accomplished using little internal "transmitters" called "oscillators" which, themselves, must be designed according to the wavelengths needed. Typically two or three stages of conversion are needed and used so that means two or three of these oscillators and the ancillary electronics to support them.

One of the chief space users in terms of components is the filter. Filters are used in multiple stages of the conversion mentioned above. So that means you need filters to handle the incoming RF energy as well as those for the intermediate converted RF energy.

Especially at the first stage, these filters can be large when handling low frequencies. You could omit them entirely but that can and does compromise the radio's performance as mentioned earlier. Many of the "teeny tiny" pocket receivers that cover all the way down to the AM broadcast band do exactly this; they can work ok as long as you are using a small inefficient antenna but will become overwhelmed when used with a good efficient tuned external antenna.

Cellular phones only have to deal with frequencies between 824MHz and about 2GHz (in the US but it's a similar range in other countries). That's still a good spread but, again, your lowest frequency being received doesn't have a wavelength much longer than a little over a foot while the scanner has to deal with wavelengths as long as 39 feet at 25MHz! Inductors, which consist of coils of wire in the simplest form, are another type of component that is very dependent on the wavelength of the energy being passed through them in terms of achieving the desired performance. You can make inductors now in surface mounted configurations that are quite small compared to the older through-hole forms and the materials science has made great strides in this area but, still, they can become quite large relative to other components when having to make them work with lower frequencies. Again, they can be made smaller to some degree but with degraded performance. Inductors are used in many RF oscillator designs so, again, you see the issue.

Another thing to understand is that cellular phones only have to handle strong to moderate signal levels in a heavily supported infrastructure dependent environment. By that I mean that the cellular service is heavily subsidized by major corporations and they spend a lot of money making sure that they have strong signals for their services present in as many areas as possible. They do this by establishing many many small transmission sites around where they expect customers to use their service - "cell sites". So a typical cellular phone will not be expected to have to handle very low level signal levels unless they are way out in the middle of nowhere away from the service areas, in which case they usually do not even guarantee any coverage. So the recievers inside these phones really do not have to be of excellent quality - only "good enough" to handle the expected service areas. Another aspect of this is the limited bands that are needed - they don't have to receive anything outside of those bands so they can have filters installed to protect them from some outside "noise sources" for example from strong signals outside the cellular bands themselves; unless you employ many expensive (and space hungry!) discreet filters that can be switched rapidly as the frequency changes this is a luxury that a wideband radio receiver may not enjoy. Even so, the filters inside cell phones, besides only needing to handle smaller wavelengths, only need to be "good enough", again, because those huge expensive infrastructure base transmitters are so prevalent that the levels of the desired signals are so strong that interference is usually just swamped by the signal you actually want. Also, those big corporations have spent a lot of money lobbying the FCC (in the US, similar agencies in other countries) to allow them to have thyeir own dedicated bands with a fair amount of guard band space between them so that interference from non-cellular services is usually very minimal; in fact, due to the nature of cellular signals in terms of their extreme prevalence and overall power levels, it is usually those non-cellular services that need to be protected from the cellular signals!

Scanner receivers must handle a very wide frequency range and signals at very low levels while potentially in the presence of undesired very strong signals (including those heavy cellular signals). There is no heavily subsidized and supported "infrastructure" for a wideband radio receiver! There are often no "guard bands" to protect the signals one might want to listen to at any given moment from such a receiver. Again, designing a good well performing wideband receiver that can handle very low level signals that a cellular phone would never be expected to deal with as well as handling very strong signals at the same time is not an easy job! Doing so and keeping the cost down is even tougher. With all of the different wavelengths of signals banging around inside a wideband receiver at various levels you have to be carefull in how you "lay out" the circuit - even things like the links between stages must be carefully considered so as to not allow signals to interfere with each other in destructive ways. In many cases, this requires - TA DA! - "space" on the circuit board. Add to this all of the customer desired digital "goodies" that can themselves generate a lot of undesired noise that has to be dealt with and you start to see the issue. Again, a cellular based smart phone can have a major digital computer section which does create noise but because we are dealing with large to moderate desired signal levels at nearly all times and virtually never with very low level desired signals we don't have to make the RF section so robust in terms of noise rejection - just "good enough".

There are many other factors of course, including the excellent ones brought up earlier by others in this thread (the cellular phone industry has A LOT of support and makes A LOT of money so components for that service have become ubiquitous and relatively cheap - not so with wideband scanner radios).

All of the above primarily concerns conventional superheterodyne stand alone receiver design. Software Defined Radio, SDR, designs are a bit of a different breed and could present some good options. If you can really get a nice analog to digital ("A/D") converter with a wide dynamic range front end (able to handle very low level desired signals while in the presence of strong undesired ones) that can simplify many design aspects in terms of the RF side of things. So it is likely that future designs for stand alone wideband scanner receivers of low cost will wind up relying on software defined radio architecture if the market does continue to support such receivers. But for the moment, even with most SDR designs of at least moderate cost, we still usually need at least one RF conversion stage before the A/D converter and usually one RF amplifier and one RF filter for best performance at reasonable cost. Even so, it is amazing what those little USB digital TV receiver sticks can do with pretty minimal front end RF electronics; just don't overload them too much!

-Mike
 
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screamin72

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We don't want some credit card sized scanner it will surely receive poorly and not enough audio. Some of us like to dx the troposphere openings. That needs a ample circuit with the sensitivity. If you like thinking scanners are stuck in the 90's then you need to be reminded....we are stuck in the early 1900's when radio was invented and it gained popularity. Radio in itself is a rather old technology. Cell phones included because they have radio technology. If it wasn't for radio cell phones won't exist.

Televisions, satellites, radio scanners, shortwave receivers, garage doors openers, wireless routers FM radios, anything that receives and or transmits is based on technology well over 100 years old. So technically we are ALL so 1900's. I enjoy it. Been into radio hobby since 1992.

Food for thought.
 

majoco

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My little Yaesu VR2 works well as a scanner and it has a 2m/70cm transmitter too. Just used as a scanner, it will run for over a week if I use it about 4 hours a day on it's internal cellphone type battery.
 
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