Digital frequency display device

A digital frequency display device includes a counter for counting a frequency to be displayed, a latch circuit for temporarily storing a plurality of digit signals from the counter and for outputting the digit signals in response to the latch pulses, a decoder for decoding the digit signals from the latch circuit into another coding system, a display section for visually displaying the digit signals decoded from the decoder, a gate for preventing the latch pulses from reaching the latch circuit at undesired times and a digit eraser for erasing any one of the digits of the numerals representing the frequency displayed when the latch pulses feeding to the latch circuit is blocked by the gate means.

BACKGROUND OF THE INVENTION 
The present invention relates to a digital frequency display device for 
communication apparatuses such as receivers or transmitters. 
Recently, communication apparatuses such as transceivers have frequently 
used a digital frequency display device for visually displaying the 
operating frequency. 
The present invention proposes several improvements over the conventional 
display device of this type. The first is an addition of a new display 
function so that, during receiving or transmitting operation, a specified 
display frequency is memorized and other frequency bands or frequencies 
may be searched or two frequencies may be alternately observed for 
experiments, with the indication of the fact that the specified display 
frequency is memorized. The second improvement is the addition of means so 
that when the communication apparatus is first turned on, the frequency 
counter portion of the apparatus for digitally displaying the display 
frequency and the memory portion for temporarily storing the display 
frequency in the digital frequency display device are simultaneously are 
not subjected to transients in operation. In conventional apparatus, the 
display frequency being counting in the counter portion is incorrectly 
and/or unstably displayed. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a digital 
frequency display device in which, during communicating operation, while a 
specified operating frequency is memorized, other frequency bands or 
frequencies may be searched or if the display of the original frequency is 
necessary, return to the original frequency is easily made. 
Another object of the present invention is to provide a digital frequency 
display device in which the memorizing condition of the display frequency 
is indicated by erasing any one of digits of the numerals representing the 
display frequency, thus eliminating any additional component for the 
memorizing indication. 
A further object of the present invention is to provide a digital frequency 
display device free from unstable and incorrect display arising from 
transient operations of the respective portions of the display device when 
the display device is first turned on. A digital frequency display device 
according to the present invention comprises a first means for storing a 
frequency to be displayed at a desired time, and a second means for 
indicating the fact that the display frequency is now memorized by the 
first means. 
In another aspect of the present invention, there is provided a digital 
frequency display device comprising a first means for storing a frequency 
to be displayed at a desired time, a second means for indicating the fact 
that the display frequency is now memorized by the first means, and a 
third means for making the first means effective in operation after a 
predetermined time. Other objects and features of the present invention 
will be apparent from the following description taken in connection with 
the accompanying drawings, in which:

In the drawings, like reference numerals are used to indicate like or 
equivalent components, sometimes with additional characters. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will be made to FIG. 1 schematically illustrating a digital 
frequency display device according to the present invention. In the 
figure, a reference frequency oscillator 10 is coupled with a gate pulse 
generator 12 which also coupled with a latch pulse generator 14 and a gate 
16. The gate 16 is connected with the output of a receiving section 20 
through a frequency mixing circuit 18. The receiving section 20 
corresponds to the one of a communication apparatus such as a receiver or 
a transmitter. The latch pulse generator 14 is connected with a latch 
circuit 26 through an AND gate 30 whose input also is connected with a 
digit eraser 22 and a switch 32. The digit eraser 22 is connected at the 
output with a decoder 24 which in turn is coupled with the display section 
21. The gate 16 is outputted to a frequency counter 28. 
The frequency mixing circuit 18 is used for forming the display frequency 
by mixing the local oscillating frequencies from the receiving section 
such as, for example, BFO and VFO frequencies. The resultant frequency 
from the frequency mixing circuit 18 is applied to the frequency counter 
28 through the gate 16. The frequency counter 28 includes a plurality of 
decimal counters (not shown) whose necessary number depends on the digits 
of the display frequency to be displayed. The coded output of the 
frequency counter 28 is applied through the latch circuit 26 to the 
decoder 24 where it in turn is decoded to drive the digital display 
section 21 also labeled Nd for frequency display. Generally, the latch 
pulse generator is directly coupled with the latch circuit 26; however, in 
this example, the AND gate 30 is used therebetween. The gate 30 as shown 
is enabled in response to the open condition of the switch 32 as shown. 
The latch circuit 26 transmits only the counting results of the frequency 
counter 28. More specifically the latch circuit 26 stores the counting 
results until the frequency counter 28 completes a given counting cycle. 
Then the stored data is transmitted to the display section 21 through the 
decoder 24 upon receipt of the latch pulse. The latch circuit 26 is also 
resetable for storing the ensuing counting results. Thus, if no latch 
pulse is applied to the latch circuit, the contents then being stored the 
latch circuit 26 will be displayed in the display section 21. The digit 
eraser 22 controls the decoder 24 to indicate the fact that the display 
device is in memorized condition. This will be described in detail later. 
In operation, when the switch 32 is open, the AND gate 30 is enabled to 
permit the latch pulses from the latch pulse generator 14 to pass 
therethrough to the latch circuit 26, thus providing digital frequency 
display in the conventional way. 
When the switch 32 is closed, the AND gate 30 is disabled and blocks 
transfer of the latch pulses to the latch circuit 26, with the result that 
the stored contents the latch circuit 26 remains unchanged and the display 
of the frequency in the display section 21 is fixed. In this situation, 
the digit eraser 22 may be driven to control the decoder 24 to erase the 
unnecessary digits of the numerals representing the display frequency. For 
example, as shown in FIG. 1, the frequency 14.237.5 (MHz) is displayed. 
Generally, the higher digits infrequenctly vary or are invariable, with 
little necessity of memorizing such digits. Accordingly, it is possible to 
erase one or two high digits of the displayed numerals for the display 
frequency to provide a memorizing indication and without reduction in the 
utility of the frequency display. Erasure of only a small number of digits 
also ensures rememberance as necessary when the digit eraser is released 
to re-display the memorized frequency. 
Referring now to FIG. 2, circuit is shown for erasing a digit of the 
displayed frequency. The switch 32 is connected through an AND gate 30A to 
an unblanking terminal b of the decoder 24 whose output is connected to a 
display element 34 also labeled LED. An incoming signal from the latch 
circuit 26 via a terminal IN is coupled with the input of the AND gate 30A 
and the decoder 24. The close of the switch 32 enables the AND gate 30A to 
drive the unblanking circuit via the terminal b so that the display 
element LED correspondingly related to the unblanking terminal b goes out. 
This circuit principle is applicable to the display elements corresponding 
to the digits necessarily to be erased in display. 
FIG. 3 shows in block form a circuit construction of a digital frequency 
display in which the present invention is applied to a dynamic drive type 
digital frequency display. This circuit uses a scanning drive circuit 23 
for driving the respective digits and a multiplexer 25. The scanning 
circuit 23 is inserted between the digit eraser 22 and the decoder 24, and 
the demultiplexer 25 is connected between the latch circuit 26 and the 
decoder 24. The output of the scanning circuit 23 is also connected 
demultiplexer 25. The erasing of the digits desired to be erased may be 
effected by omitting the scanning of the corresponding digits through the 
control of the scanning circuit 23. 
Turning now to FIG. 4, another embodiment of the digital frequency display 
device according to the present invention is shown. The receiving section 
20 is connected to the frequency mixing circuit 18 which in turn is 
connected to the gate 16. The gate pulse generator 12 is connected to the 
gate 16 and the latch pulse generator 14 which, in turn, is connected to 
the AND gate 30. Upon receipt of the gate pulse 12, the gate 16 is enabled 
to allow the resultant frequency from the frequency mixing circuit 18 to 
pass therethrough to the counter 28. The reference frequency oscillator 10 
is coupled to the gate pulse generator 12 and a scanning control circuit 
23A. The scanning control circuit 23A essential to this embodiment is 
responsive to the output of the oscillator 10 to cause the multiplexer 38 
and the demultiplexer 40 to do scanning operations. When the switch 32 is 
closed, the indication of the memorized condition is provided by the 
display device. The multiplexer circuit 38 is used to successively and 
selectively extract one by one the digit outputs temporarily stored in the 
latch circuit 26A. The demultiplexer 42 successively and selectively 
demultiplexes the outputs of digital signals from the decoder 40 to apply 
them to the corresponding display elements of the display section 21. The 
gate 30 electrically disconnects the latch circuit 26A from the latch 
pulse generator 14, when the switch 32 is closed. The scanning control 
circuit 23A selectively interrupts the scanning operation of the 
multiplexer and demultiplexer 38 and 42 when the switch 32 is closed. 
In operation, when the switch 32 is open, the latch pulses from the latch 
pulse generator 14 is fed to the latch circuit 26A, and the digital 
display operation is similar dynamic drive mode. More specifically, the 
display frequency via the counter gate 16 from the frequency mixing 
circuit 18 is fed to the frequency counter 28 where it is counted. In 
response to the latch pulses from the latch pulse generator 12, the digit 
outputs a to f from the counter 28 are temporarily stored in the latch 
circuit 26A. These stored digits outputs stored are successively and 
selectively extracted one by one from the latch circuit 26A through the 
multiplexer to be applied to the decoder 40 where they are decoded into 
the corresponding digital display signals. Those digital display signals 
are successively and selectively taken out of the decoder 40 to be fed to 
the corresponding display elements of the display section 21. With this 
arrangement the counted frequency is correctly displayed in the display 
section 21 and the displayed frequency will change to reflect changes in 
the counted frequency. That is, the counted frequency to be displayed is 
transmitted to the display section every time the latching pulse is fed to 
the latching circuit 26A. 
When the switch 32 is closed, the AND gate 30 is disabled to disconnect the 
latch circuit 26A from the latch pulse generator 14. As a result, 
frequency values stored immediately before the switch 32 is closed are 
kept stored in the latch circuit 26A and the display displays fixedly such 
frequency values. It is further seen that, the closing of the switch 14 
drives the sweep control circuit 23 which in turn controls the sweeping 
operations of the multiplexer 38 and the demultiplexer 42 to interrupt the 
display of at least one of the digit outputs of the latch circuit 26A. The 
result is that the display element is blanked, providing an indication 
that the digital frequency display device is now in the memorized 
condition. 
FIGS. 5 and 6 provide further details of the scanning control circuit 23A. 
In FIG. 5, a modulo-6 counter 510 clock pulse input of which is obtained 
by frequency-dividing the reference frequency from the reference 
oscillator 10. A switching transistor 530 is connected at the base with 
the switch 32 through a resistor, at the collector with one of inputs of a 
NAND gate 520, and at the emitter with the input terminal A of a BCD to 
decimal code decoder 500. The NAND gate 520 is coupled at another input 
with the output terminal Qc of the modulo-6 counter 510 and the input 
terminal C of the BCD to decimal code decoder 500, and at the output with 
the modulo-6 counter 510. The modulo-6 counter is connected with the BCD 
to decimal code decoder 500 through corresponding terminals. The switching 
state of the transistor 530 is controlled by the ON/OFF operation of the 
switch 32 as shown. The BCD to decimal code decoder converts the BCD code 
outputs of the modulo-6 counter 510 into the decimal code signal which is 
used as a scanning signal for the multiplexer 38 and the demultiplexer 42. 
When the switch 32 is open, the BCD outputs of the modulo-6 counter 510 are 
code-converted in the BCD to decimal code decoder 500 into the decimal 
code signals which in turn appear at the output terminals 0 to 5 to be 
provide to the multiplexer 38 and the demultimplexer 42. Therefore, all of 
the display elements of six digits are lighted. 
When the switch 32 is closed, the switching transistor 530 is turned off 
thereby forcing the modulo-6 counter 510 to operate as a modulo-4 counter. 
As a result, no output appears at the output terminals 4 and 5 of the BCD 
to decimal code decoder 510 so that the scanning operations of the 
multiplexer 38 and the demultiplexer 42 are partially interrupted and two 
of the six display elements are not lit up. 
Another scanning control circuit is shown in FIG. 6 in which a combination 
of three NAND gates 610, 620, and 630 and a switch 32 A are used in place 
of the combination of the switching transistor 530 and the switch 32 in 
the circuit of FIG. 5. In this instance, when the moveable element of the 
switch 32A is connected to the lower side contact as shown, the counter 
510 operates as the modulo-6 counter. However, when it is connected to the 
upper side contact, the counter 510 is reset to operate as the modulo-5 
counter, and one display element cannot then be lighted. 
It will, of course, be understood that conventional shift-registers and 
Johnson counters could be used in the scanning control circuit 23A. 
FIGS. 7 and 8 illustrate further features of the digital frequency display 
device according to the present invention such as the use of a delay 
circuit 31. Other circuit components are the same as those previously 
mentioned. As previously, the outputs of local oscillators (not shown) 
from the receiving portion 20 are mixed in the mixing circuit 18 to form 
the frequency to be displayed. The gate pulses from the gate pulse 
generator 12 derived from the reference frequency generator 10 enables the 
gate 16 to permit the frequency to be counted and displayed to pass 
therethrough to the frequency counter 28. The latch pulse generator 14 
also connect to the gate pulse pulse generator 12 feeds latch pulses to 
the latch circuit 26 through the gate 30 which is controlled by the switch 
32. The display frequency values from the counter 28 are temporarily 
stored in the latch circuit 26 and fed to the display section 21 through 
the decoder 24. 
As in the previous cases, when the switch 32 is turned off, the AND gate 30 
is enabled so that the latch pulses from the latch pulse generator 14 are 
applied to the latch circuit 26 and the frequency display operation occurs 
in a conventional manner. On the other hand, when the switch 32 is turned 
on, the gate 30 is disabled, thereby preventing the latch pulses from 
being applied to the latch circuit 26. As a result, the contents of the 
latch circuit 26 at the time of are memorized, and the displayed frequency 
is frozen (displayed fixedly). 
The delay circuit 31 in FIG. 7 delays the turn-off signal from the switch 
32 to the AND gate 30 when the power switch of the communication apparatus 
including the receiving portion 20 is turned while the switch 32 is 
already turned on. This avoids transient operation and causes the digital 
frequency display to be stable, thereby ensuring the correct frequency 
display. 
FIG. 8 shows a schematic circuit diagram for the delay circuit 31 of FIG. 
7. A power supply voltage +B is provided with value sufficient to enable 
the AND gate 30 through a resistor R1 when the switch 32 is turned off 
(open). A switching transistor Q1 is provided for executing the on-off 
control of the input of the AND gate 30, through cooperation with resistor 
R1 is a load resistor cooperating with the switching transistor Q1 to drop 
the collector voltage of the transistor Q1 to a satisfactory value when 
the transistor Q1 is turned on. Resistors R2 and R3 provide a bias voltage 
to the transistor Q1 when the switch 32 is turned on (closed). A capacitor 
C1, in cooperation with a resistor R3, provides a time constant circuit 
controlling the bias voltage to the transistor Q1. This arrangement 
prevents the transistor Q1 from turning on until after the power supply 
voltage, i.e. the +B voltage, has risen to a predetermined level after a 
predetermined time delay. By such time the receiver operation will have 
become stable, thereby permitting the frequency to be counted and 
displayed correctly without transient disturbances. 
In operation, when the switch 32 is open, the base-collector circuit of the 
transistor Q1 is inversely biased and the transistor Q1 is turned off. 
Accordingly, the +B voltage is applied to the control input of the AND 
gate 30 through the resistor R1 and the AND gate 30 is enabled when the 
latch pulse is applied to the other input of the AND gate 30, thereby 
permitting the latch pulse to pass therethrough to the latch circuit 26. 
When the switch 32 is closed, the emitter-base circuit of the transistor Q1 
is forwardly biased to turn-on the transistor, thereby effectively 
grounding one of inputs to the AND gate 30 so that the AND gate 30 is 
disabled to electrically disconnect the latch circuit 26 from the latch 
pulse generator 14. 
When the power switch of the communication apparatus is first turned on, 
the +B voltage rises from zero over a predetermined time interval to final 
value +B. Meanwhile the capacitor C1 is charging to provide the bias 
voltage to the transistor Q1. Once the bias voltage across the capacitor 
C1 has reached a predetermined value, and if the switch 32 is then turned 
on, the transistor Q1 will be turned on instantaneously, permitting a 
correct frequency display operation. 
When the power switch is first turned on, the initial value of the +B is 0. 
If the switch 32 is also turned on, the base voltage of the switching 
transistor Q1 will increase to a predetermined level after a time interval 
determined by the capacitor C1 and the resistor R3, and the transistor Q1 
will then turn on. 
Incidentally, the time constant set by the capacitor C1 and the resistor R3 
is preferably established to be comparable to a several, such as three, 
counting cycles of the frequency counter. 
Various other modifications of the disclosed embodiments will become 
apparent to persons skilled in the art without departing from the spirit 
and scope of the invention as defined by the appended claims.