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Frequency accuracy

Discussion in 'General Electronics Discussion' started by Paul Jefferies, Jan 10, 2016.

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  1. Paul Jefferies

    Paul Jefferies

    7
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    Jan 10, 2016
    Hi Folks,

    I am new to the forum and though I do have a Ham licence, my knowledge of electronics and amatuer radio is pretty sketchy. I have recently dug out an old Icom 718 which has been in storage for some years. I have two quite different frequency meters and I find that on 14MHz the transmitter is transmitting 1KHz high on one meter and 3KHz high on the other....... Is this "acceptable" and if not, is there anything I can do about it?

    Thanks in advance,

    Paul
     
  2. Martaine2005

    Martaine2005

    3,599
    979
    May 12, 2015
    Hi Paul,
    I don't know anything about Ham radio, but found this site which could answer your question.
    Read it all or scroll to the bottom for your 14 mhz question.

    Martin
     
  3. Richard Ferguson

    Richard Ferguson

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    Jan 10, 2016
    Hi Paul,

    I'm not familiar with the 718 but if it covers 15 Mhz, you could "zero beat" the 15 Mhz WWV transmission in either USB or LSB mode and see how close your display reads to 15Mhz. That might tell you which of the two meters or your radio is most accurate. If you can't tune to 15 Mhz you can try one of the other WWV frequencies. (5 or 10 Mhz). Be sure t do your zero beating during the last 15 seconds of the minute when there is no tone on their transmission or you will get an incorrect zero.

    This will tell you where the error is. Perhaps all three units need adjusting (the radio as well as both meters). If you determine that at least one of them is correct, you can then focus on the problems one at a time.

    In the mean time, just plan on staying away from the band edges. Give yourself a 10 Khz margin until you figure out what's going on.

    Rich, NN7D
     
  4. Paul Jefferies

    Paul Jefferies

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    Jan 10, 2016
    Thanks for your replies. Unfortunately it seems there are no longer any Standard Frequency Transmissions in Europe and so far I have not been able to receive WWV. Martin, in the article you pointed me to they seem to suggest that 3Khz is the maximum acceptable error. Richard, I will heed your suggestion to stay away from the edge of the bands and once I get an antenna working properly, I will ask any contacts what frequency they think I am on.........

    73s

    Paul
     
  5. Richard Ferguson

    Richard Ferguson

    4
    1
    Jan 10, 2016
    Hi again Paul,

    According to what I can find, your 718 has general coverage receive down to 30 Khz. I should think there would be an AM broadcast station near you that would serve as a frequency reference. A talk radio station would be easier because there would likely be brief periods of time during a persons speech when there was no audio being transmitted and you could clearly hear the stations carrier signal. Again, just tune for a zero beat and compare your radio's frequency display with the radio stations assigned carrier frequency. I don't know about your area but stations in this country are held to a very tight tolerance and frequency of operation is very accurate. Other frequency references might include certain types of electronic equipment or games or a television set. Be especially careful around TV sets though. Older models have some very dangerous voltages inside. If you have a personal computer there is a very stable oscillator in there. These devices aren't intended to radiate but they all do a little. If you run an "INSULATED" wire from your radio's antenna port to the inside of your computer ( don't connect it to anything. In fact make sure there is no bare copper exposed on your wire, use a small piece of electrical tape to cover the end of the wire where it was cut) you will hear signals in there. Try to use an older desk top PC if available. You're more likely to find a lower frequency oscillator in the older models. You may have to hunt around a little to locate the oscillator and hopefully, it will have the frequency marked on it.

    Your best bet at this time, however, is probably a broadcast AM radio station. There are lots of options when it comes to frequency references. You haven't said anything about your two frequency meters but my guess is that your 718 is the most accurate. You could probably take it to someone and have it checked out but one of the biggest pleasures in life is learning how to do things for yourself.

    Good Luck,

    Rich NN7D
    Flagstaff, Arizona
     
  6. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    I had a look at what the Icom 718 is. It is a transmitter/receiver running on the normal HF bands. It uses digital signal processing with several filters and frequency offsets so I do not understand how it can be old. How the frequency is displayed will need to be determined.

    The frequency transmitted may perhaps be obtained by listening to it on the receiver at the secret bunker near Nantwich. I see no advantage in running close to the band edges.

    WWV is intermittent in the UK, you just have to wait for propagation conditions to be suitable.

    If the counters differ, then one or both are wrong. Have you listened on 5MHz or 10MHz for other frequency standards?
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    11,826
    2,757
    Nov 17, 2011
    You could tune to the DCF77 time signal in Frankfurt, Germany and compare it to a mixed-down version (/180) of your 14MHz signal, although, since 14MHz is not an integer multiple of 77.5kHz you'll have to take into account the beat frequency. Or, if you can, tune the 14MHz oscillator to 14.0275MHz for an integer multiple. Once you have established this relationship you can compare the readings of the two f-meters to find out which one is more accurate.
     
  8. Paul Jefferies

    Paul Jefferies

    7
    0
    Jan 10, 2016
    Gentlemen, thank you again for taking the trouble to reply...... Richard, as I understand "zero beating", I would need some kind of oscillator and I don't have one. I have scanned around the AM stations and found several but the 718 seems to receive each station at anything up to 4Kcs either side of the nominal frequency. As for my frequency counters, one is a Galaxy II model FC250 which I bought off ebay maybe 12 years ago......
    Frequency counter.JPG
    This one is within about 1Kc of the Transmitter's readout.

    The other was also bought off ebay just last year and is one of these.......
    Frequency Counter 2.JPG
    This one reads about 3Kc higher than the 718's readout

    I realise that neither is exactly "Top of the Range" and could be out by a few Kcs but the reason for my original question was to ask if the discrepancies would be considered acceptable and IF the counters are correct, do I need to do something about it or can I just continue to use the transmitter?

    Duke, the reason I described the 718 as "old" was that I bought it second-hand off ebay about 11 years ago, it worked well enough back then but since then I have moved house and the transceiver has not been used for several years...... but neither has the first frequency counter so the whole lot is a bit of an unknown quantity! Yes, I did listen on 5Mhz and 10Mhz but received nothing on either frequency.

    ".....one of the biggest pleasures in life is learning how to do things for yourself."............. I couldn't agree more! But I could never be described as an avid enthusiast, I am more of a "dabbler" but I don't want to break any rules!

    Paul
     
    Last edited: Jan 11, 2016
  9. Paul Jefferies

    Paul Jefferies

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    Jan 10, 2016
    Harald, thanks for your suggestion........ I have now tried the DCF77 signal and it was audible over quite a wide band. However it was loudest at around 78.3Kcs on the transceiver's display. This would suggest that my transceiver is receiving about 0.800Kcs lower than it says on the dial and if my frequency counters are correct, transmitting a little higher than it should........ Does that make sense?

    Paul
     
  10. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    You have an oscillator in your counter and another in the frequency reference. Beating these together will give an indication of the frequency difference.
    In the good old days of Radio 4 200kHz I could set up a 100kHz crystal oscillator to give a beat of longer than 10 seconds on a simple radio. Not a bad result, accurate to 1 in 10^6.

    I have a Matsui MR 4099 portable radio which is sythesised. This beats with 5Mhz or 10Mhz transmitters with the BFO set centrally, accurate to 1 in 10^7. Do you want better than this? Try using a Class D wavemeter which is adequate for a transmission measurement.:)

    Your second picture shows that both displays are the same, just that the counter has one more digit.
     
  11. Paul Jefferies

    Paul Jefferies

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    0
    Jan 10, 2016
    Duke, thanks for your suggestion, I will see if I can make that work.

    The second picture is not of my actual frequency counter. The picture was taken from the ad on ebay so the small handheld and the readings in the picture were simply a part of the advertisement. I sent off for and received one exactly the same and it is installed in a case so can't really be seen which is why I "borrowed" that picture......... Sorry for any confusion.

    Paul
     
  12. Richard Ferguson

    Richard Ferguson

    4
    1
    Jan 10, 2016
    Hi Paul,

    An AM radio transmitter transmits a central carrier signal and two sidebands. The sidebands contain all of the intelligence in the transmitted signal but the carrier signal uses a large part of the transmitters power and is required by the AM receiver as a reference to demodulate the information in the sidebands. Only one of the sidebands (either one) is required,since they both contain the same information. Since an AM receiver uses the transmitters carrier signal as its demodulation reference, you can tune an AM receive to any frequency relatively close to the center frequency and easily understand what is being transmitted. If you eliminate the transmitters carrier signal and one of the sidebands, you save a large percentage of the power required to transmit the original AM signal but the remaining sideband becomes unintelligible when listening with a standard AM receiver. The intelligence is still there however and if you re-insert a carrier signal at the correct frequency, the the transmitted signal becomes intelligible again. This is exactly what is done in a single sideband system. You start out with a standard AM signal, eliminate one or the other of the two sidebands as well as the carrier signal and only transmit the remaining sideband. If you decide to transmit the sideband on the high side of the carrier frequency, you are transmitting an "UPPER SIDEBAND" signal The alternative would be the "LOWER SIDEBAND". The carrier, then needs to be re-inserted at the receiver end of the system. This process saves a tremendous amount of power at the transmitting end but greatly increases the complexity of the receiver.

    When you listen to an AM signal with a SSB receiver, you essentially have one sideband ( the other sideband is filtered out in your receiver) and two carrier signals, the carrier transmitted by the radio station and the locally generated carrier generated inside your SSB receiver. Your receiver hears the AM transmitters carrier just like any other radio signal and that carrier signal will mix with the receivers locally generated carrier in a circuit called the product detector and produce a tone in your speaker. The frequency of that tone in your speaker is equal to the difference in frequency between the broadcast stations carrier frequency and the frequency of the locally generated carrier frequency inside your receiver. If the broadcast station is transmitting on 93 KHZ and your receiver is tuned to 92 Khz and you have selected Lower Sideband, you will hear a 1Khz tone in your speaker. If you slowly retune your receiver closer and closer to 93 Khz, the difference between the two carrier signals decreases and the "DIFFERENCE" tone goes down in frequency until the two carriers are exactly the same frequency at which point the "DIFFERENCE" frequency goes to "ZERO" and you receiver is said to be "ZERO BEAT" with the transmitters carrier frequency. If your receiver is properly aligned, the frequency display on your radio will be the same as the transmitted frequency of the broadcast transmitter.

    Older model receivers used band switching and complex mixing schemes with many separate oscillators to accomplish this Since they had several oscillators involved, it was necessary to align the oscillators for each band individually so that the frequency display on your radio would be correct on all bands. On an older radio like that you couldn't zero beat a signal on 93 Khz and know that if your receiver displayed 93 KHZ, that it would also be accurate when listening to a signal on 14 MHZ because when you changed your receiver to listen to 14 Mhz, you were selecting a different crystal oscillator inside your receiver to serve as the basis for your locally generated carrier.

    Modern radios use a different method of generating all of these frequencies which only relies on one crystal oscillator. In this case if your radio displays the correct frequency at 93 Khz, it will more than likely display the correct frequency at 14 Mhz as well. This isn't absolutely true all the time, but you can pretty much depend on it.

    In your statement, you indicated that you thought you needed another oscillator to "ZERO BEAT" with the local AM radio station. You are correct and the point of this discussion is to let you know that you already have that oscillator built in to your radio.

    Your locally generated carrier oscillator will not only mix with the radio stations carrier oscillator, it will also mix with the sidebands transmitted by the radio station. Only when your radio is tuned to the correct frequency will the audio coming from your receiver sound normal to you. Normal sounding speech is also an indication of "ZERO BEAT". In fact, some communications receivers use filters to eliminate the very low audio tones and make it very difficult to listen for a good zero beat. In this case, you may have to turn the audio volume up very high to hear the zero beat or just say "This is good enough, if I'm within a few Hz, I'm happy"

    Rich NN7D
    Flagstaff, Arizona USA.
     
    hevans1944 likes this.
  13. Paul Jefferies

    Paul Jefferies

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    Jan 10, 2016
    Rich, thank you so much for your clear and patient explanation. As I read it I realised just how much I had forgotten in the years since I was doing this kind of thing! However, untill you mentioned it in an earlier reply, I had not heard of "zero beating" and after a little googling I had misunderstood what was involved. After reading your post I went out and tried it again on two separate frequencies...... The Frankfurt Time signal on 77.5 Khz as suggested by Harald and the BBC on 198Khz and it worked just great" As far as I can tell my transceiver is just about spot on! :)

    I am embarassed to have to admit that the reason I thought it was transmitting a bit high would have been because I was measuring the frequency of the Upper Sideband.:rolleyes: Transmitting on AM, the older frequency counter now agrees with the transmitter almost exactly and the cheap Chinese one is less than 1Kc out so I think we can say "Problem solved"........ The only problem was me! Thanks to all for your help and your patience!

    Paul
     
  14. davenn

    davenn Moderator

    13,903
    1,971
    Sep 5, 2009
    Paul you are mis-reading your freq counter it's reading 300 Hz high NOT 3kHz high

    xxx.2253
    2 = 100's of kHz
    2 = 10's of kHz
    5 = units of kHz
    3 = 100's of Hz
    and unless that handheld was tested on a calibrated freq counter, you have no idea if that 300 Hz is an error in the counter or in the radio OR a bit of both !!!

    Dave
     
  15. Paul Jefferies

    Paul Jefferies

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    Jan 10, 2016
    Dave, please read my posting three down from the pictures in which I explain that the picture was taken from the ebay listing for my meter. Mine is identical to that one but is not that actual one. The purpose of the pictures was to show the TYPES of meters I have been using...... just in case everybody except me knows that one or other of them is rubbish!

    Paul
     
  16. hevans1944

    hevans1944 Hop - AC8NS

    4,631
    2,159
    Jun 21, 2012
    @Paul Jefferies: And then there is the other method (NOT recommended) that many hams appear to use today: transmit where you will and wait for someone to tell you "You're outta da band, Lid!" Of course you wouldn't hear them because they are transmitting inside the ham band whereas you are transmitting and listening outside the band. Not a reliable method.

    If you listen carefully near the band edges, you may come to the conclusion that some hams deliberately work "outside the band" to avoid QRM. Perhaps they are hoping the FCC won't notice. Guess what? With modern automated monitoring technology, the FCC will notice. If not the FCC, then one of those ARRL Official Observers (OOs) will notice and gift you with a postcard. The NSA will definitely notice (they listen everywhere to everything, DC to Light), but they won't tell you: it just goes into your classified dossier. So, yes, it's a good idea to periodically zero-beat your transceiver against a commercial AM broadcast station, and not work too close to the band edges. Thanks to @Richard Ferguson (Rich NN7D) you now know how. Don' forget to center the RIT (Receiver Incremental Tuning) control before tuning for zero-beat.

    BTW, I envy you your Icom 718! That's a nice rig. Of course it doesn't hold up against my Elecraft KX3 if I put the KXPA shoes on, but the Icom is one nice tried-and-true package.:D I usually operate QRP because of the global warming thing.:cool: Try running less than five watts and you will be amazed at the new QRP friends you meet on the air.

    Hop AC8NS
     
    davenn likes this.
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