Dan's Radio

CB Radio

High Performance CB

  Here is an SDR spectrum display of a radio I have worked on to open up the modulation with the typical swing modulation so many desire (controlled carrier modulation where the carrier can be suppressed) however there was no attempt before this test to improve the modulation in terms of spectrum bandwidth response, however what this shows is that there is bass energy below 300 Hz and highs extending a little over 4 kHz.  This displays the typical voice frequency response of the radio.  The water fall display beneath the spectrum display provides a visual display of events recorded in time.  Near the bottom of the display you will see bass energy in red on the 100 and 200 Hz scales.  With a source such as music you will see more bass energy below 300 Hz as well as highs extending to around 4.5 kHz.   This test was done using an RTLSDR hardware receiver with HDSDR software and the Galaxy DX 2547 loaded into a 50 ohm dummy load, modulating on 27.215 MHz.   Below we will look at one of the methods one can employ to improve the bass response as well as sensitivity and bandwidth characteristics of this radio's modulation.

  Above is a test circuit set up depicting the Galaxy DX 2547 modulation coupling section, with the load resistance equivocated to 2.5k ohms, which is approximate, give or take a few hundred ohms which in no wise effects the dB range of the analysis since halving the load or doubling the load only has a 3 dB effect.  The lower circuit in the dashed box is the circuit in the radio, while the one on top is redesigned to act as a low pass audio filter that is down -6 dB @ 5 kHz, while also boosting the sensitivity of the audio by +3 dB as well as increasing the bass end of the audio spectrum.  The effect can be seen in the plotted analysis of the two circuits side by side in the following plot.

  As can be seen in this plot the redesigned circuit in green has increased bass end response with a 3 dB gain in audio sensitivity as compared to the original circuit plotted in white.  This gain helps when resorting to using he stock hand mike, but in no wise makes the radio too wild for use with a power mike, the gain in effect helps within the bass region of the audio.  Also as can be seen in the green plot the circuit now acts as an audio bandpass filter reducing the spectrum to -6 dB down at 5 kHz which conforms to a 6 dB bandwidth which is pretty standard in terms of bandpass.  Furthermore the spectrum created by this modification is ideal for use on sideband, while it improves the modulation quality on AM.  And it helps to keep the modulation from reaching over into the adjacent right and left channels on the band, as long as the ALC is adjusted properly to maintain some nice sine wave response without modulation clipping (flat topping of the modulation).

  This is a kind of minimalistic design modification that improves the performance of the modulation section.  Similar modifications are done in the microphone amplification section of the Galaxy radio to provide an over all performance throughout the modulation audio chain.

  This describes some of the many modifications that goes into my radios which also includes the typical bells and whistles, such as uppers and lowers, etc.

   And of course I will not divulge all of my circuit designs, for proprietary trade secret reasons of which you will understand.