ksbszabo@watvlsi.UUCP (Kevin Szabo) (01/09/86)
I have found that 99% of auto AM radios use a bank of variable inductors for tuning; whereas all other portable AM's use variable capacitors. Anybody out there know why? My only guess is that the variable capacitors might not be rugged enough, but it seems unlikely. Kevin Szabo -- Kevin Szabo' watmath!watvlsi!ksbszabo (VLSI Group, U. of Waterloo, Ont., Canada)
alw@mit-eddie.UUCP (Alan Wu) (01/11/86)
In article <2859@watvlsi.UUCP> ksbszabo@watvlsi.UUCP (Kevin Szabo) writes: >I have found that 99% of auto AM radios use a bank of variable >inductors for tuning; whereas all other portable AM's use >variable capacitors. Anybody out there know why? My only >guess is that the variable capacitors might not be rugged >enough, but it seems unlikely. I remember reading some 10-15 years ago (in a book about vacuum tubes) that the reason variable inductors were used in automotive radios was their superior resistance to vibration and better stability in spite of extreme temperature changes. This was in comparison to air-dielectric capacitors (it was an old book even when I read it). I'm not sure why modern variable capacitors aren't used today -- perhaps it's tradition or perhaps there's still a temperature drift problem. -- --Alan Wu UUCP: alw@mit-eddie.uucp ARPA: alw@eddie.mit.edu.arpa
dsi@unccvax.UUCP (01/11/86)
Aside from ruggedness considerations, one benefit of inductor
tuned AM radio front ends is their compactness. I have seen complete
inductor tuned front ends, including RF amplifier, mixer, and oscillator
done in about the space of 2 28 pin DIP's stacked on top of each other
It is also easier to wind the coils with variable pitch (so that the
displacement of the slug in the inductor represents a constant frequency v.
distance relationship on the dial) but evidently many manufacturers forget
this. In fact, Ford Motor Co (variously also known as Aeroneutronic Ford
and Ford Refrigeration and Entertainment Division) doesn't stop with the
AM section. There is a "fourth" slug tuned circuit which is
an oscillator. However, this oscillator drives a frequency
discriminator providing the FM tuning diode voltage.
Here the same bennies apply, such as constant dial displacement
versus frequency. At FM, you get the added bonus of not having micro-
phonics in the audio due to rattling tuning capacitors.
An inductor tuned front end also provides a better impedance match
to the type of antenna found in the car (very short monopole) which
means a better S+N ratio.
David Anthony
DataSpan, Inccrs@lanl.ARPA (01/13/86)
> >inductors for tuning; whereas all other portable AM's use > >variable capacitors. Anybody out there know why? My only > >guess is that the variable capacitors might not be rugged > >enough, but it seems unlikely. > > I remember reading some 10-15 years ago (in a book about vacuum tubes) > that the reason variable inductors were used in automotive radios was > their superior resistance to vibration and better stability in spite > of extreme temperature changes. This was in comparison to air-dielectric > capacitors (it was an old book even when I read it). I'm not sure why > modern variable capacitors aren't used today -- perhaps it's tradition > or perhaps there's still a temperature drift problem. This is just a guess but... The variable capacitors used for tuning radios have plates whose shape and position on the shaft are quite eccentric. I would guess that with such a configuration, the constant vibration would cause the thing to rotate thus changing the setting. -- The opinions expressed are not necessarily those of my employer, the government or your favorite deity. Charlie Sorsby ...!{cmcl2,ihnp4,...}!lanl!crs crs@lanl.arpa
larry@kitty.UUCP (Larry Lippman) (01/13/86)
> I have found that 99% of auto AM radios use a bank of variable > inductors for tuning; whereas all other portable AM's use > variable capacitors. Anybody out there know why? My only > guess is that the variable capacitors might not be rugged > enough, but it seems unlikely. I suspect that the use of slug-type inductors facilitates the mechanical design of the pushbotton preset tuner. While there are piston-type capacitors, they are generally used for higher frequencies than AM broadcast band (since the capacitance range available is typically << 100 uuF); in addition, I would guess that a piston-type capacitor is more expensive to manufacture than a comparison inductor. ==> Larry Lippman @ Recognition Research Corp., Clarence, New York <== ==> UUCP {decvax|dual|rocksanne|rocksvax|watmath}!sunybcs!kitty!larry <== ==> VOICE 716/741-9185 {rice|shell}!baylor!/ <== ==> FAX 716/741-9635 {G1, G2, G3 modes} duke!ethos!/ <== ==> seismo!/ <== ==> "Have you hugged your cat today?" ihnp4!/ <==
dma@ssc-vax.UUCP (Dennis Anderson) (01/14/86)
> I have found that 99% of auto AM radios use a bank of variable > inductors for tuning; whereas all other portable AM's use > variable capacitors. Anybody out there know why? My only > guess is that the variable capacitors might not be rugged > enough, but it seems unlikely. > > Kevin Szabo > -- > Kevin Szabo' watmath!watvlsi!ksbszabo (VLSI Group, U. of Waterloo, Ont., Canada) It probably allows for a more reliable push-button tuning system. Small errors in the position of a tuning slug would have less effect on the inductance than the same amount of slop in the angle of a variable capacitor. Dennis Anderson @ Boeing Aerospace
crs@lanl.UUCP (01/17/86)
> >It probably allows for a more reliable push-button tuning system. Small errors >in the position of a tuning slug would have less effect on the inductance than >the same amount of slop in the angle of a variable capacitor. WHY? -- The opinions expressed are not necessarily those of my employer, the government or your favorite deity. Charlie Sorsby ...!{cmcl2,ihnp4,...}!lanl!crs crs@lanl.arpa